[bugfix]for division use accurate division

for now / just get int value ,this will lead method
filter_destination_hosts return 0 host
because self.threshold / 100 * host.vcpus is 0
so we need use accurate division to change this

close-bug: 1689269

Change-Id: I5663951ce750d6c4580a507ccfc0268baea0685f

watcher/decision_engine/strategy/strategies/workload_balance.py

@@ -46,6 +46,7 @@ hosts nodes.
   algorithm with `CONTINUOUS` audits.
 """
 
+from __future__ import division
 import datetime
 
 from oslo_config import cfg