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
Add extra extension file to makemessage command line

specify the file extensions of the makemessage to *.html, *.txt, *.csv

Closes-Bug: #1226910
Change-Id: I115cd448249ac0b1b27a6b28fca3f16d6b2670a6
Merge "Handle the case no SimpleTenantUsage Nova extension"
Merge "Add extra extension file to makemessage command line"