adminport: url-prefix checking and log-prefix.

Change-Id: I1f921365920b0dbf48e76f77962ea23317ac0ac5
Reviewed-on: http://review.couchbase.org/44164
Reviewed-by: Sarath Lakshman <sarath@couchbase.com>
Reviewed-by: Pratap Chakravarthy <prataprc@couchbase.com>
Tested-by: Pratap Chakravarthy <prataprc@couchbase.com>