LU-2010 tests: replay-single/44a,b,45 find only tested mdc

If multiple filesystems are mounted, then test_45 would have found
mdcdevs for each filesystem.  This corrects that to only look for the
filesystem that is being tested against.

Also correctly delete old test files.

Signed-off-by: Nathaniel Clark <nathaniel.l.clark@intel.com>
Change-Id: I0297cf9b21e9c97ee468cd81d5d4ac409c59d1e0
Reviewed-on: http://review.whamcloud.com/4724
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Jian Yu <jian.yu@intel.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>