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: Ia82518ecfe4c96bca18595a12d5763ff1bd00721
Partial-Bug: PROD-17878
2 files changed