dynamic text for 'stop rebalance/failover' button

Change-Id: Ic01189ee503036f3d986e5df3ebe05aea69759a3
[alk@tut.by: adjusted rest api]
Signed-off-by: Aliaksey Kandratsenka <alk@tut.by>
Reviewed-on: http://review.couchbase.org/37181
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>