MB-5307: bucket warming up on some node(s) is not error

I.e. we're lowering log message severity which better reflects real
world.

Change-Id: I1a5d2e985a2f718ac8da5d2d6a3843b7a389f96d
Reviewed-on: http://review.couchbase.org/23596
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
MB-5307: allow 'rebalance' with specified vbucket moves

So that we can exercise some specific moves for sake of benchmarking
and testing. As well as some cases of near-disaster recovery.

Change-Id: I87e0378daf0668c7c88530ba2b6b6845e0871861
Reviewed-on: http://review.couchbase.org/23597
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>