Handle stale instance metadata

[mogren]

Issue #1008

Description of changes:
The CNI needs to handle inconsistent IMDS metadata on startup. Added retries and better error handling. The EC2 API will only return the ENI ID of the first ENI that doesn't exist in the error message, so we have to keep retrying. Added more test cases.

Tests
Two non-existing ENIs added:

{"level":"info","ts":"2020-06-06T00:39:58.944Z","caller":"runtime/asm_amd64.s:1357","msg":"Synced successfully with APIServer"}
{"level":"info","ts":"2020-06-06T00:39:58.944Z","caller":"k8sapi/discovery.go:216","msg":"Add/Update for CNI pod aws-node-glbkt"}
{"level":"error","ts":"2020-06-06T00:39:58.960Z","caller":"ipamd/ipamd.go:328","msg":"Failed to call ec2:DescribeNetworkInterfaces for [eni-043a9bb8856789abc eni-00000099 eni-00000088]: InvalidNetworkInterfaceID.NotFound: The networkInterface ID 'eni-00000099' does not exist\n\tstatus code: 400, request id: 192e237e-dbb4-46d3-bcc1-1e184ddd19cf"}
{"level":"debug","ts":"2020-06-06T00:39:58.960Z","caller":"ipamd/ipamd.go:328","msg":"Could not find interface: The networkInterface ID 'eni-00000099' does not exist, ID: eni-00000099"}
{"level":"error","ts":"2020-06-06T00:39:59.037Z","caller":"ipamd/ipamd.go:328","msg":"Failed to call ec2:DescribeNetworkInterfaces for [eni-043a9bb8856789abc eni-00000088]: InvalidNetworkInterfaceID.NotFound: The networkInterface ID 'eni-00000088' does not exist\n\tstatus code: 400, request id: 45415af0-835d-49c1-9b60-dc235eeeacd5"}
{"level":"debug","ts":"2020-06-06T00:39:59.037Z","caller":"ipamd/ipamd.go:328","msg":"Could not find interface: The networkInterface ID 'eni-00000088' does not exist, ID: eni-00000088"}
{"level":"debug","ts":"2020-06-06T00:39:59.114Z","caller":"ipamd/ipamd.go:314","msg":"DescribeAllENIs success: ENIs: 1, tagged: 0"}
{"level":"info","ts":"2020-06-06T00:39:59.114Z","caller":"ipamd/ipamd.go:360","msg":"Setting up host network... "}

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

[jayanthvn]

Looks good to me :)