MB-7249 added case insensetive check for bucket names conflict

...to the bucket creation validation REST API

Change-Id: I9acfcefde400a2fe81ed8639ee65b9cea01cc4f9
Reviewed-on: http://review.couchbase.org/27252
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Tested-by: Artem Stemkovski <artem@couchbase.com>