LU-15410 tests: Add MDS Space check for dom-performance

LU-15410 tests: Add MDS Space check for dom-performance

IOR Test within dom-performance requires at least
MDS of 20GB. This patch adds MDS space check for
dom-performance/test_IOR to skip in case the MDS
of required size is not found

Test-Parameters: trivial testlist=dom-performance
Signed-off-by: Arshad Hussain <arshad.hussain@aeoncomputing.com>
Change-Id: I493a6ee5b549539b562aeda418a7418b94060ca9
Reviewed-on: https://review.whamcloud.com/45973
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: James Nunez <jnunez@whamcloud.com>