Name various entities on GlusterFS backend with "manila-" prefix

On the course of their operation, the GlusterFS drivers (glusterfs and glusterfs-native) allocate various named entities in the backing GlusterFS cluster.

Currently it's done in a very direct way, basically just using the UUID of the Manila entity on behalf we create the Gluster entity to name the latter. To make intospection easier for the Gluster admin, these entities should rather have a "manila-" prefix in their name.

At the moment this occurs in the following cases:

- in the glusterfs driver, when creating a share, a top level directory is created on backend;-
- in the glusterfs-native driver, when creating a snapshot, a snapshot is created on the Gluster backend as well.

Changing these to apply the "manila-" prefix is trivial; the harder part is backward compatibility which we should maintain.

Apart from changing what's already there, this blueprint also intends to pin down naming policy for similar cases in the future.