Document servicegroup flag in Nova

The blueprint has been implemented in Grizzly and this patch improve the documentation
about this new feature in Nova.

Blueprint : memcached-service-heartbeat
Fix Bug #1181984
Cherry-picked from https://review.openstack.org/#/c/29752/

Change-Id: Ib75f42d45fcbf0b8d02d874342b27fb28c8309c7