mgr_util: add CephfsClient implementation

mgr_util: add CephfsClient implementation

This pulls parts of the VolumesClient implementation into mgr_util to
make the CephFS specific pieces available to other mgr modules. To
reduce code duplication the VolumeClient now extends the CephfsClient
class to add the volume specific methods.

Signed-off-by: Jan Fajerski <jfajerski@suse.com>
(cherry picked from commit a44de38b61d598fb0512ea48da0de4179d39b804)

	src/pybind/mgr/mgr_util.py
	src/pybind/mgr/tox.ini
	src/pybind/mgr/volumes/fs/operations/volume.py
	src/pybind/mgr/volumes/fs/volume.py
  Trivial conflicts because ofthe order of backports to octopus