expose user name in Creds interface

So that we can audit user actions

Change-Id: Idc4d410b08ad828d66e79d7b9b9425636ce80b33
Reviewed-on: http://review.couchbase.org/43974
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>