re-factor roles validation code into a proper validator
re-factor roles validation code into a proper validator

Change-Id: I783aea250afccfd02763ffb118c60e1d458f6444
Reviewed-on: http://review.couchbase.org/89301
Well-Formed: Build Bot <80754af91bfb6d1073585b046fe0a474ce868509@couchbase.com>
Tested-by: Artem Stemkovski <15a0730a27c69f62ab86f2b6c639fca202166c02@couchbase.com>
Reviewed-by: Aliaksey Artamonau <ce14b1c16e0fd654f926118eb8addbec5aff4ffa@couchbase.com>

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