Add leader_utils:live_nodes/{1,2,3}.
Add leader_utils:live_nodes/{1,2,3}.

In the orchestration context, the node being live usually means that
it's not failed over and it's not down from Erlang distribution
perspective. The function name is a bit too generic to put in
elsewhere.

Change-Id: Ibfe681bcc497a2a327cf3109c83f293b818baf62
Reviewed-on: http://review.couchbase.org/89575
Reviewed-by: Poonam Dhavale <6855f09986fe28ed7e8cdd9d60db878d898703d5@couchbase.com>
Tested-by: Aliaksey Artamonau <ce14b1c16e0fd654f926118eb8addbec5aff4ffa@couchbase.com>

membase/ns_server,membase/ns_server,membase/ns_server,membase/ns_server,membase/ns_server,membase/ns_server