make the pgbackrest stanza parameterizable

[mzwettler2]

The stanza for pgbackrest is hardcoded with "db" (stanza=db).

In many cases it would be helpful if the stanza could be parameterized, e.g. when using one s3 bucket for multiple pg clusters.
currently the user has to separate them by another repo path.

With a parameterizable stanza, a unique repo name could be easily configured:

metadata.name: .Values.clusterName
pgbackrest.stanza: .Values.clusterName

[ValClarkson]

Hi @mzwettler2
thank you for submitting this issue.
I've added an item to our development backlog.