task/cephfs: generalise Filesystem for multi-MDS

This enables tasks like mds_journal_migration to be
run in an environment with standby-replay MDSs present.

Signed-off-by: John Spray <john.spray@redhat.com>
task/workunit: clean up dir deletion

This was always trying to remove the mountpoint,
and then swallowing the exception and printing
it to the log.

Instead:
 * Ensure it's only trying to delete mnt if it created it
 * Don't swallow those exceptions: if deleting the dirs
   fails then something has gone wrong and it should bubble
   up.

Signed-off-by: John Spray <john.spray@redhat.com>
task/workunit: general tidy

* PEP8-ize whitespace
* specialize a catch-all except:
* don't alias 'misc' to 'teuthology'

Signed-off-by: John Spray <john.spray@redhat.com>