LU-3313 tests: Skip sanityn test_51b

sanityn test_51b now checks the metadata server version and
will run the test for versions 2.3.59 and above. The test will be
skipped for all other server versions.

Signed-off-by: James Nunez <james.a.nunez@intel.com>
Change-Id: I040a77a3564e4801413ff342519ba10c54583e70
Reviewed-on: http://review.whamcloud.com/6340
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Jian Yu <jian.yu@intel.com>
Reviewed-by: Keith Mannthey <keith.mannthey@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>