Make sure we have space for a trunk ENI

[mogren]

What type of PR is this?
Bug fix

Which issue does this PR fix:
#1202

What does this PR do / Why do we need it:
We should only set the vpc.amazonaws.com/has-trunk-attached label on the node if there is space to attach one more ENI.

Testing done on this change:
Manual test in a cluster with MAX_ENI=1:

{"level":"debug","ts":"2020-09-13T00:13:38.120Z","caller":"ipamd/ipamd.go:481","msg":"No slot available for a trunk ENI to be attached. Not labeling the node"}
{"level":"debug","ts":"2020-09-13T00:13:40.620Z","caller":"ipamd/ipamd.go:476","msg":"Reconciling ENI/IP pool info because time since last 2562047h47m16.854775807s <= 1m0s"}
{"level":"debug","ts":"2020-09-13T00:13:40.621Z","caller":"ipamd/ipamd.go:912","msg":"Total number of interfaces found: 1 "}

After removing the MAX_ENI setting, the label gets applied, and a trunk ENI attached:

{"level":"info","ts":"2020-09-13T00:15:29.736Z","caller":"ipamd/ipamd.go:836","msg":"Updated node ip-192-168-198-30.us-west-2.compute.internal with label \"vpc.amazonaws.com/has-trunk-attached\": \"false\""}
{"level":"debug","ts":"2020-09-13T00:15:32.240Z","caller":"ipamd/ipamd.go:836","msg":"Node label \"vpc.amazonaws.com/has-trunk-attached\" is already \"false\""}
{"level":"info","ts":"2020-09-13T00:15:34.862Z","caller":"ipamd/ipamd.go:946","msg":"Updated node ip-192-168-198-30.us-west-2.compute.internal with label \"vpc.amazonaws.com/has-trunk-attached\": \"true\""}

Automation added to e2e:
None. Standard e2e tests pass: https://app.circleci.com/pipelines/github/mogren/amazon-vpc-cni-k8s/963/workflows/34c4ef50-c137-4ea8-a1b1-039964bf7cd0/jobs/1950

Will this break upgrades or downgrades. Has updating a running cluster been tested?:
No

Does this change require updates to the CNI daemonset config files to work?:
No

Does this PR introduce any user-facing change?:
No

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.