Use max function for openstack_* related metrics

If we use an aggregate func like max, we will avoid the
'multiple series' error, which can happen if for the specific
time period there will be >1 instance of remote agent returned
by the metric.

Change-Id: I27ce7eb1de7c63c6fa1609f89c0ded60f60bc039
Partial-Bug: PROD-17878
2 files changed