[backups] Allow specify the secret keys if an existing secret is created.

[ercpereda]

When you pass the following values:

backups:
  secret:
    create: false
    name: my-existing-secret

Forces my-existing-secret to contain ACCESS_KEY_ID and ACCESS_SECRET_KEY.
The name of those keys are hardcoded in the template (See ACCESS_KEY_ID and ACCESS_SECRET_KEY)

This could be useful in some sceneries. For instance, when you use Rook(Ceph) to create a Bucket a secret is created with the credentials, right now isn't possible to refer to that secret because the name of the keys doesn't match with the expected by this chart.