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.

In addition, "set_default_debug_nodes()" has a defect to check
whether "$nodes" contains the local node or not. The patch contains
a fix for that.

Signed-off-by: Yu Jian <yujian@whamcloud.com>
Change-Id: Iae49497601d09fef38968360d8543cc753009e85
Reviewed-on: http://review.whamcloud.com/871
Tested-by: Hudson
Reviewed-by: Brian J. Murrell <brian@whamcloud.com>
Reviewed-by: Chris Gearing <chris@whamcloud.com>
Reviewed-by: Johann Lombardi <johann@whamcloud.com>