MB-4345 Move ns_config_isasl_sync to ns_server_sup.

I want it to be stopped and started again when
ns_server_cluster_sup:{stop,start_cluster} are called
correspondingly. This way ns_config_isasl_sync will always use
up-to-date admin credentials.

Change-Id: Ia7fb6222a338eca20a2dc930e01962149f8e2222
Reviewed-on: http://review.couchbase.org/29069
Tested-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>