MB-10415 ability to do auth using memcached password

... for some special cases like moxi

Change-Id: I248ffb8ba70912886ddfe9136c63137570f3e1b5
Reviewed-on: http://review.couchbase.org/35426
Tested-by: Artem Stemkovski <artem@couchbase.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>