qa/cephfs: backup mount object before running tests

qa/cephfs: backup mount object before running tests

Right now, only client IDs are stashed and restored but with the recent
changes (addition of more attributes to mount objects, specifically),
this is not enough. Saving and restoring these details before and after
tests respectively ensures that mount commands rus smoothly. Not doing
this typically leads to mount command failure for the second test in the
testsuite under execution since the client IDs are saved and restored in
CephFSTestCase.setUp and CephFSTestCase.tearDown respectively but the
rest of the details are not.

Signed-off-by: Rishabh Dave <ridave@redhat.com>