feat(node affinity): add support to specify node affinity rules of NFS Server

[mittachaitu]

Pull Request template

Why is this PR required? What issue does it fix?:
This PR adds support to specify node affinity rules via NFS-Provisioner
ENV to schedule NFS Server on a specific set of nodes. It fixes #57

What this PR does?:
This PR adds a feature to propagate node affinity rules to NFS Server
instance via Provisioner ENV.

Does this PR require any upgrade changes?:
No

How to use?:

• Add 'OPENEBS_IO_NFS_SERVER_NODE_AFFINITY' ENV in NFS-Provisioner deployment in following manner:

  - name: OPENEBS_IO_NFS_SERVER_NODE_AFFINITY
    value: "kubernetes.io/hostname:[172.17.0.1],kubernetes.io/os:[linux]"

• To schedule NFS Server instance on storage & nfs nodes

  - name: OPENEBS_IO_NFS_SERVER_NODE_AFFINITY
    value: "kubernetes.io/storage,kubernetes.io/nfs-node"

  Ex: Propagation to NFS Server deployment

  ...
  ...
  ...
         nodeAffinity:
         requiredDuringSchedulingIgnoredDuringExecution:
           nodeSelectorTerms:
           - matchExpressions:
             - key: kubernetes.io/storage-node
               operator: Exists
             - key: kubernetes.io/arch
               operator: Exists

How it is propagated to NFS Server instance:

• During boot-up time of provisioner instance, provisioner will read
  OPENEBS_IO_NFS_SERVER_NODE_AFFINITY ENV then parse
  affinity rules and store them under the affinity rules in form of Go structure[in-memory].
• When volume is provisioned NFS-Provisioner will propagate this affinity
  rules to NFS Server instance.
  Example propagation:

...
...
...
      affinity:
        nodeAffinity:
          requiredDuringSchedulingIgnoredDuringExecution:
            nodeSelectorTerms:
            - matchExpressions:
              - key: kubernetes.io/hostname
                operator: In
                values:
                - 172.17.0.1
              - key: kubernetes.io/os
                operator: In
                values:
                - linux

If the changes in this PR are manually verified, list down the scenarios covered::

Any additional information for your reviewer? :
Mention if this PR is part of any design or a continuation of previous PRs

Checklist:

• Fixes support to provision nfs-volume using the node information provided via storageclass or env in provisioner #57
• PR Title follows the convention of <type>(<scope>): <subject>
• Has the change log section been updated?
• Commit has unit tests
• Commit has integration tests
• (Optional) Are upgrade changes included in this PR? If not, mention the issue/PR to track:
• (Optional) If documentation changes are required, which issue on https://github.com/openebs/openebs-docs is used to track them:

Signed-off-by: mittachaitu sai.chaithanya@mayadata.io

[codecov-commenter]

Codecov Report

Merging #59 (5adecb5) into develop (e0a9e18) will decrease coverage by 11.53%.
The diff coverage is 44.64%.

@@             Coverage Diff              @@
##           develop      #59       +/-   ##
============================================
- Coverage    48.37%   36.83%   -11.54%     
============================================
  Files           18       27        +9     
  Lines         1598     2234      +636     
============================================
+ Hits           773      823       +50     
- Misses         781     1366      +585     
- Partials        44       45        +1     

Impacted Files	Coverage Δ
...tes/api/core/v1/podtemplatespec/podtemplatespec.go	37.23% <0.00%> (-4.64%)	⬇️
provisioner/helper_kernel_nfs_server.go	0.00% <0.00%> (ø)
provisioner/provisioner.go	0.00% <0.00%> (ø)
provisioner/node_affinity.go	96.07% <96.07%> (ø)
provisioner/env.go	14.28% <100.00%> (ø)
provisioner/signal_handler.go	0.00% <0.00%> (ø)
provisioner/provisioner_kernel_nfs_server.go	0.00% <0.00%> (ø)
... and 4 more

---

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[kmova]

@mittachaitu -- is it feasible to modify the schema for specifying the affinity a litte for the cases where there no values. for example is it possible to just use:

- name: NODEAFFINITY
  value: "kubernetes.io/storage"

or a comma separated values like this:

- name: NODEAFFINITY
  value: "kubernetes.io/storage, openebs.io/nfsnodes"

[mittachaitu]

@mittachaitu -- is it feasible to modify the schema for specifying the affinity a litte for the cases where there no values. for example is it possible to just use:

- name: NODEAFFINITY
  value: "kubernetes.io/storage"

or a comma separated values like this:

- name: NODEAFFINITY
  value: "kubernetes.io/storage, openebs.io/nfsnodes"

Question: As of now we are supporting only In & Exists operations. Do we also need to support NotIn & DoesnNotExist cases? If yes without values how can we take negate input?

If we follow with key-value pairs even though there is no value then-current implementation is easy to extend for negation. For example negation config

- name: NODEAFFINITY
  value: "kubernetes.io/application:~[], kubernetes.io/zones:~[zone-2,zone-3]"

can be converted to

            - matchExpressions:
              - key: kubernetes.io/application
                operator: DoesNotExist
              - key: kubernetes.io/zones
                operator: NotIn
                values:
                - zone-2
                - zone-3

[kmova]

This should be merged after #58 - as there are some helm chart files with conflicting changes.