This patch adds a config option 'mount_point_base' by which we can
provide the mount point for *fs volumes.
The different types of FS drivers will have separate directory for
mounting their volumes,
Eg: <mount_point_base>/nfs, <mount_point_base>/xyzfs etc.
The default mount point for *fs volumes is /var/lib/glance/mnt/*fs.
Closes-Bug: #1866966
Change-Id: I10b5ea4943ad9388ce1962eae9204b905c8ff24a
(cherry picked from commit 1a656163c9)
c6b7be68f8