[leader_activities] Add missing 'follower' quorum to the type.
[leader_activities] Add missing 'follower' quorum to the type.

It's supported by the code, but is not mentioned in the quorum()
type.

Change-Id: I3f2c9ca649d8b8d1c79529d4c6547aa58c48fbca
Reviewed-on: http://review.couchbase.org/89726
Tested-by: Aliaksey Artamonau <ce14b1c16e0fd654f926118eb8addbec5aff4ffa@couchbase.com>
Reviewed-by: Artem Stemkovski <15a0730a27c69f62ab86f2b6c639fca202166c02@couchbase.com>

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