LU-2353 tests: Introduce back end mount/unmount helpers

LU-2353 tests: Introduce back end mount/unmount helpers

Some tests need to access back end file systems directly.  Currently,
for ldiskfs targets, they either mount the devices as ldiskfs or
invoke the ldiskfs-specific debugfs tool; for ZFS targets, they simply
skip forward.  Many of them could actually work on both back end
types.  Since the mount/unmount procedures diff for ldiskfs and ZFS
back ends, this patch introduces a pair of generic helpers,
mount_fstype()/unmount_fstype(), to hide the differences from tests.
Then, sanity 27z is adapted to work with ZFS targets using the new
helpers, as an example.

Test-Parameters: mdsfilesystemtype=zfs ostfilesystemtype=zfs mdtfilesystemtype=zfs
Change-Id: I0b6b358b147240519f28852ba2c8fea85c09aab3
Signed-off-by: Li Wei <wei.g.li@intel.com>
Reviewed-on: http://review.whamcloud.com/5785
Tested-by: Jenkins
Reviewed-by: Nathaniel Clark <nathaniel.l.clark@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Jian Yu <jian.yu@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>