MB-8665: killed stopRebalanceIsSafe flag on pool details

It was used to see if we need to ask user "cluster is probably
partitioned are you sure to stop rebalance?". But it wasn't very
correctly implemented (stopRebalanceIsSafe change wasn't causing pool
details refresh correctly) and inherently racy.

New UI will just send stop rebalance request with special flag
"onlyIfSafe". And if rebalance is not safe it'll signal error causing
UI to display confirmation. After confirming it'll send stop rebalance
request without the flag.

Change-Id: If334fbe92eaec99fe9b631eda21dc6cd5832670d
Reviewed-on: http://review.couchbase.org/28883
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>