Cherry-pick #24912 to 7.x: Kubernetes_secrets provider improvements

[ChrsMark]

Cherry-pick of PR #24912 to 7.x branch. Original message:

Minor leftovers from #24789.

1. Return from Fetch() if k8s client is not initialised. This can happen if the provider failed to get started with Run() cause api can be unreachable. In such cases Fetch() will then panic. We need to skip Fetch() in such cases.
2. Add the commented out section in Agent's k8s manifests about Secrets api.

[elasticmachine]

Pinging @elastic/integrations (Team:Integrations)

[elasticmachine]

Pinging @elastic/agent (Team:Agent)

[ChrsMark]

This one also adds some missing parts of the manifests that were missed in the original backport #24231.

Both backports are for 7.13 so we are on the safe side here.

[elasticmachine]

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• Build Cause: Pull request #24968 updated

• Start Time: 2021-04-07T13:09:21.955+0000

• Duration: 95 min 41 sec

• Commit: 78d9ee2

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Total	6588

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Test	Results
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Passed	6564
Skipped	24
Total	6588