MB-100 allow free form Destination in ns_connection_pool

there's no need to require it to be {Host, Port, Ssl}

Change-Id: Iec30b5a308855af3f6e1bc31965e4f8c635915d0
Reviewed-on: http://review.couchbase.org/32747
Tested-by: Artem Stemkovski <artem@couchbase.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>