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
(cherry picked from commit d43e49ae465c501ff453364a7c7cbe5f0ec8a0f0)

Conflicts:

	openstack_dashboard/dashboards/project/overview/tests.py
Add volume limits to project overview.

Commit d4b0ab4aa395bf4df2964efcc358100117efdaa0 fixing
bug #1178694 removed volume quotas from project overview.
Cinder is not reporting Snapshot limits today, so that info
can't be added without including quotas again, defeating the
purpose of the original fix.
This fix will only add Volume Qty & Total Storage GB used (including
snapshot size, if any ).
Information is hidden if volume list is empty.

https://review.openstack.org/#/c/56488/3

Partial-Bug: #1244875
Change-Id: Ia72e40329ecf08b57fb26cd847f02cececc53579
(cherry picked from commit 5cfaa81c4bf04c6fb67977e4aad7849603144cb3)

Conflicts:

	horizon/templates/horizon/common/_limit_summary.html
	openstack_dashboard/dashboards/project/overview/tests.py