mgr: create `volumes` module

mgr: create `volumes` module

This encapsulates and extends ceph_volume_client, providing
similar functionality as a service from ceph-mgr.

We used to call CephFS namespaces "filesystems", and the
ceph_volume_client-created directories "volumes".  That
terminology changes in this module: namespaces are now "volumes",
and the directory-based entity is a "subvolume".

External systems can use librados to access the command
interface provided by this module, instead of using
ceph_volume_client directly.

Signed-off-by: John Spray <john.spray@redhat.com>