Comment 3 for bug 1412856

Making rbd cache settings controllable via charm makes sense for the reason
stated above but also to allow caching to be disabled for Ceph >= Giant since
some deployments may not want it to be enabled, perhaps for consistency when
performing an upgrade. Question is do we enable or disable by default in the
charms. I propose that we use an option like the following to allow Ceph
default behaviour to be used:

rbd-client-cache:
  type: string
  default:
  description: |
    Enable/disable rbd client cache. Leaving this value unset will result in
    default Ceph rbd client settings being used (rbd cache is enabled by
    default for Ceph >= Giant). Supported values here are "enabled" or
    "disabled".