mgr/volumes: implement subvolume based on subvolume template

mgr/volumes: implement subvolume based on subvolume template

subvolume base class implements common routines/helpers and
initializes a metadata manager. later, when v2 subvolume version
is implemented, the metadata manager would be used to persist
subvolume metadata in ceph filesystem. this would allow flexible
metadata management when complex subvolume features are added.

typically, a subvolume would be implemented by subclassing the
subvolume base class and the subvolume template -- instantiating
this would be called a "subvolume object".

with this commit, current subvolume topology is maintained. but
we introduce the concept of subvolume versions. a loader stub
loads available "versions" of subvolumes. right now, the only
available version is v1. since backward compatibility needs to
be maintained for existing subvolumes, the loader API allows
version discovery w/ auto upgradation to the most recent version.

Signed-off-by: Venky Shankar <vshankar@redhat.com>