MB-4345 Use unique admin memcached password on each node.

Change-Id: I0cc69e5b77c0b7aae20bf4a30d6c01186f1fd34e
Reviewed-on: http://review.couchbase.org/28950
Reviewed-by: Artem Stemkovski <artem@couchbase.com>
Tested-by: Artem Stemkovski <artem@couchbase.com>