LU-4749 mgs: Recognize colons in failover.node values

LU-4749 mgs: Recognize colons in failover.node values

Current mgs assumes that all the NIDs in a single failover.node
parameter belong to one node.  This is not the case, even prior to
http://review.whamcloud.com/11161, because mkfs.lustre options like

  --servicenode=<nid1>,<nid2>:<nid3>,<nid4>

are allowed.

Change-Id: If3e2ebc0e81093e9c8304e496afdca24edf456ef
Signed-off-by: Li Wei <wei.g.li@intel.com>
Reviewed-on: http://review.whamcloud.com/11956
Tested-by: Jenkins
Reviewed-by: Johann Lombardi <johann.lombardi@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Liang Zhen <liang.zhen@intel.com>