Minor fixups for username/bucket username

- Undocument username for now (until we actually have a 'read-only'
  user)
- For tests, specify admin_username and admin_password

Change-Id: I7b77feb49416e4740c6c181c95c86899f1d2ede3
Reviewed-on: http://review.couchbase.org/28779
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Matt Ingenthron <matt@couchbase.com>
Reviewed-by: Volker Mische <volker.mische@gmail.com>