MB-10260: expose rebalance status request timeout in tasks

So that clients are able to distinguish if rebalance is actually not
running or if we simply failed to reach ns_orchestrator

Change-Id: Ic413b7ab453be10fdf763e31dc2f18fbc94563b5
Reviewed-on: http://review.couchbase.org/42212
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>