Block ELB re-creation after the cluster has been provisioned

[Ankitasw]

What type of PR is this?
/kind feature

What this PR does / why we need it:
Since ELB is created only when it's not found, this PR blocks the ELB creation in the scenario where if user deletes ELB post the cluster has been provisioned. The reason to do this is if ELB is created again after the cluster has been provisioned, then CAPA will create new ELB with new DNS endpoint which won't work.
As AWSClusterSpec.ControlPlaneEndpoint becomes immutable after cluster is provisioned, we are relying on the validity of this endpoint to know the status of provisioning of the cluster to check whether ELB has to be created or not. If this endpoint is valid and ELB is not found, then we inform the user that AWSCluster is in failed state and unrecoverable.

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 #2608

Checklist:

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

Release note:

Block ELB re-creation after cluster is provisioned.

[k8s-ci-robot]

@Ankitasw: This issue is currently awaiting triage.

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[Ankitasw]

/test pull-cluster-api-provider-aws-e2e

[Ankitasw]

/test pull-cluster-api-provider-aws-e2e

[sedefsavas]

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: sedefsavas

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