MB-10213 use admin credentials to connect to memcached during rebalance

this will prevent data corruption caused by connecting to wrong memcached
due to incorrect /etc/hosts file

Change-Id: Ib0ed568382dfac229ad6a628a3e4224bd3dca6e1
Reviewed-on: http://review.couchbase.org/36884
Tested-by: Artem Stemkovski <artem@couchbase.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>