MB-100:replaced extract_auth(username, ..) with explicit function

Change-Id: I6b3e17b634342ab28fd96b4a1df615f90a17aaf3
Reviewed-on: http://review.couchbase.org/27945
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>