With cinder configured as glance store and nfs as cinder backend,
if we create multiple images concurrently into the same share,
there might be race conditions.
This patch ensures a lock per export to avoid the race conditions.
This patch also introduces a reference counting mechamism for unmounting
which will unmount the share if no active thread/process is using it.
Closes-Bug: #1870289
Change-Id: I9197f64e29a0ae2e0a58186f1a70aa134f7f1db6
f5d4699613