The new helper supports to optionally retrieve common
configuration options for BlueStore Compression.
Fix ordering of Ceph class composition. Both the CephCharm and the
fake charm class used in the unit test had the wrong order for the
composition making it impossible to override already existing
methods inherited from OpenStackCharm.
Change-Id: I9fb543f297522c7217acdcfa950bb2810b26c4a0