Trick dialyzer

The cb_auth_info module is only there so we can run couchdb without
the full Couchbase stack. Currently cb_auth_info just returns some
authentication credentials. Dialyzer does complain about it:

    couch_set_view_group.erl:3792: The pattern {'error', 'server_not_ready'}
    can never match the type {'auth',<<_:80>>,<<_:96>>}

Hence I added some code to trick dialyzer so that it doesn't emit
any warnings.

Change-Id: Ia8bc7ed40e6211d768cb0d21aad8b533e9495c00
Reviewed-on: http://review.couchbase.org/35242
Reviewed-by: Filipe David Borba Manana <fdmanana@gmail.com>
Tested-by: buildbot <build@couchbase.com>