Document recommended way of passing secrets to Functionbeat

[kvch]

Currently, the keystore in Functionbeat should not be used. When the function is deployed to the cloud provider, the keystore cannot be opened, as it has too restrictive permissions. We should suggest users to use the recommended way of sharing secrets with functions: environment variables. As the keystore command right now is not useful, it would be nice to not to include it in the docs of Functionbeat.

See more: #15808 (comment)

[dedemorton]

Closed by #16010