Add nova/cinder quotas management in benchmark context

This patch allows to modify quotas related to nova and cinder for
generated benchmark's tenants by extended the context object,
allowing to run large scale scenarios.

Blueprint benchmark-context-tenant-quotas

Change-Id: Ia21ce2494018d9ac466ede6a87729554f2b547f8
Merge "Add nova/cinder quotas management in benchmark context"