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

New password is generated every time config is upgraded. That is,
on fresh nodes, during offline upgrade and after node is ejected from
a cluster.

Also on node eject memcached to pick up new credentials it has to be
explicitly restarted.

Change-Id: I58822405b4468698d8d4830c4c44cfb77c726735
Reviewed-on: http://review.couchbase.org/29017
Tested-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>