Getting a secret content from the controller is quite the expensive
operation, cache the secret for the duration of the hook to speed up
hook execution.
cache the `access-credentials` secret from the ceph-access relation
cache the `credentials` secret from the identity-credentials relation
Partial-Bug: #2065866
Change-Id: I4d82baf9a41ccff1129f462f3a00676baf9d6d5e
712f666b30