db, core, webadmin: Add is_live_merge_supported flag

Some hosts may not have updated software capable of live merging
snapshots, which vdsm will report in its capabilities using the
'liveMerge' flag.

Capture this flag, and use it to limit live merge operations on hosts
not supporting the feature.

Change-Id: I932a74f4010b8b256f8c0cd27a2279b9d83efbb9
Bug-Url: https://bugzilla.redhat.com/1124099
Signed-off-by: Greg Padgett <gpadgett@redhat.com>