MB-10515: made 2.5.1 nodes take over mastership over 2.5.0 nodes

I.e. in order for MB-10220 fixes to be effective.

Change-Id: Ic74bd90cd5e8d7f0edb1e7386c06baad898440b2
Reviewed-on: http://review.couchbase.org/34729
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Merge remote-tracking branch 'origin/for-rackaware' into master

* origin/for-rackaware:
  MB-10515: made 2.5.1 nodes take over mastership over 2.5.0 nodes

The change is empty because 3.0 is advertising 3.0 as mb_master
version.

Change-Id: I4432b7955eeb6d0cba9068a32c531e0d049edab9