Handle the case no SimpleTenantUsage Nova extension

When the SimpleTenantUsage Nova extension is not enabled,
panels that include usages produce an error and
the usage table is useless.

Three panels use the SimpleTenantUsage extension:
- openstack_dashboard.dashboards.admin.overview
- openstack_dashboard.dashboards.admin.projects
- openstack_dashboard.dashboards.project.overview

This fix avoids errors and does not show the usage
table in such a case.

A context variable 'simple_tenant_usage_enabled' is now
available in templates rendered by usage.UsageView subclasses.

Unit tests now mock nova 'extension_supported' API.

Change-Id: Ib306846bf6c947572ba0e7c773125d03b3dbf68b
Closes-Bug: #1211470
Merge "Handle the case no SimpleTenantUsage Nova extension"