[leader_activities] Make add_activity/6 use add_activity/2.
[leader_activities] Make add_activity/6 use add_activity/2.

The code duplication is minor, but still.

Change-Id: I3c6248c82f3eda042a87f60db24d53428579670d
Reviewed-on: http://review.couchbase.org/89579
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