allow parameterizing upr vbucket move limit

I.e. some folks may want to make it higher to make rebalance
faster. There's also ongoing investigation about slow rebalance where
playing with this might tell us something useful.

Change-Id: I47310da9c613d0a09b88726985c5abba7334b01f
Reviewed-on: http://review.couchbase.org/37799
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Artem Stemkovski <artem@couchbase.com>