LU-372 add $TMP/mgsactive to indicate the active combined MGS/MDS node

LU-372 add $TMP/mgsactive to indicate the active combined MGS/MDS node

The current test-framework could not figure out the active MGS node
correctly while the MGS and MDS nodes are combined under a failover
configuration. We need add a $TMP/mgsactive file to indicate the
active MGS node like what we did for the MDS node.

Signed-off-by: Yu Jian <yujian@whamcloud.com>
Change-Id: I74bbf9293f9a290573c4b9fcbf9fea0fcc0aad95
Reviewed-on: http://review.whamcloud.com/913
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Brian J. Murrell <brian@whamcloud.com>
Reviewed-by: Chris Gearing <chris@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>