fix: resolve secrets when generating eks userdata (backport #4285 to 2.1)

[faiq]

What type of PR is this?

What this PR does / why we need it:

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #

Special notes for your reviewer:

Checklist:

• squashed commits
• includes documentation
• adds unit tests
• adds or updates e2e tests

Release note:

resolves ContentFrom for eksconfig userdata

[Ankitasw]

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: Ankitasw

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[Ankitasw]

/cherry-pick release-2.0

[k8s-infra-cherrypick-robot]

@Ankitasw: #4289 failed to apply on top of branch "release-2.0":

Applying: fix: resolve secrets when generating eks userdata
Using index info to reconstruct a base tree...
M	bootstrap/eks/controllers/eksconfig_controller.go
M	bootstrap/eks/controllers/eksconfig_controller_reconciler_test.go
Falling back to patching base and 3-way merge...
Auto-merging bootstrap/eks/controllers/eksconfig_controller_reconciler_test.go
Auto-merging bootstrap/eks/controllers/eksconfig_controller.go
CONFLICT (content): Merge conflict in bootstrap/eks/controllers/eksconfig_controller.go
error: Failed to merge in the changes.
hint: Use 'git am --show-current-patch=diff' to see the failed patch
Patch failed at 0001 fix: resolve secrets when generating eks userdata
When you have resolved this problem, run "git am --continue".
If you prefer to skip this patch, run "git am --skip" instead.
To restore the original branch and stop patching, run "git am --abort".

In response to this:

/cherry-pick release-2.0

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