This closes a bug concerning multi-registry configurations
for Quobyte volumes due to no longer using the is_mounted()
method that failed in that case.
Besides, this adds exception handling for the unmount call that
is issued on trying to mount an already mounted volume.
NOTE: The original commit also added a new feature (fs type
based validation) which is omitted in this backport.
Closes-Bug: #1737131
Change-Id: Ia5a23ce1123a68608ee2ec6f2ac5dca02da67c59
(cherry picked from commit 05a73c0f3a)
(cherry picked from commit 656aa1cd40)
c958ad8a68