feat(nfs-server): change shared filesystem ownership and mode

[niladrih]

Why is this PR required? What issue does it fix?:
Resolves: #122
This PR allows a user to change the ownership and file mode parameters of the filesystem shared using NFS.

What this PR does?:
This PR introduces the FilePermissions cas.openebs.io/config PersistentVolumeClaim key. The data keys included within this config key are UID, GID and mode. The openebs-nfs-provisioner process sets the values from these PersistentVolumeClaim keys into the ENVs FILEPERMISSIONS_UID, FILEPERMISSIONS_GID and FILEPERMISSIONS_MODE.

The /nfs-server-container/nfsd.sh script issues chmod and chown commands to change the ownership and file mode of the shared filesystem directory (backend volume at /nfsshare). The checking criteria is similar to the Kubernetes fsGroupChangePolicy OnRootMismatch.

NOTE: This also generates logs which announce the deprecation of the FSGID cas.openebs.io/config option in future releases. FSGID-like changes can be accomplished with FilePermissions. Instructions for this have been furnished in the user documentation(#128).

If 'FSGID' is specified, and 'GID' and/or 'mode' FilePermissions keys are also specified, this is treated as an invalid input and provisioning fails. This is done to keep the file permissions strictly declarative.

Sample PersistentVolumeClaim:

kind: PersistentVolumeClaim
apiVersion: v1
metadata:
  name: secure-vol
  annotations:   
    cas.openebs.io/config: |
      - name: FilePermissions
        data:
          UID: "1000"
          GID: "2000"
          mode: "0744"
spec:
  storageClassName: openebs-kernel-nfs
  accessModes:
    - ReadWriteMany
  resources:
    requests:
      storage: 10Gi

Does this PR require any upgrade changes?:
No.

[codecov-commenter]

Codecov Report

Merging #125 (dc70a26) into develop (3ce5a7d) will increase coverage by 0.32%.
The diff coverage is 52.68%.

@@             Coverage Diff             @@
##           develop     #125      +/-   ##
===========================================
+ Coverage    49.79%   50.12%   +0.32%     
===========================================
  Files           38       38              
  Lines         2705     2789      +84     
===========================================
+ Hits          1347     1398      +51     
- Misses        1255     1285      +30     
- Partials       103      106       +3     

Impacted Files	Coverage Δ
provisioner/provisioner_kernel_nfs_server.go	0.00% <0.00%> (ø)
provisioner/config.go	31.51% <54.79%> (+21.61%)	⬆️
provisioner/helper_kernel_nfs_server.go	72.23% <100.00%> (+0.81%)	⬆️

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[mittachaitu]

Looks good but good to have the following things:

• Integration test for a new feature
• Documentation and helm changes as separate PR(also update vanilla YAML's about new feature with comments). If we have a document along with this PR then users may assume feature exists in a released version which don't want to happen, so good to have different PR for release related work.

[mittachaitu]

Provided few minnor comments

[niladrih]

This PR depends on #129 .

[mittachaitu]

LGTM