add ImageId to the set of additional cloud provider labels

[justinmir]

What type of PR is this?
/kind feature

What this PR does / why we need it:
Introduces new label applied to each node in a cluster: k8s.aws/image-id=.

This gives cluster administrators better visibility into images being used by nodes
in their cluster.

It does not require any additional API calls as the describe instance call already has the ImageId.

Which issue(s) this PR fixes:
Fixes #1118

Special notes for your reviewer:
N/A

Does this PR introduce a user-facing change?:

Export image id as an node label: `k8s.aws/image-id=`

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