Compute v2.1 docs clean up (part 6) (not linked wadl)
Compute v2.1 docs clean up (part 6) (not linked wadl)

Add extensions and os-simple-tenant-usage
  it is based on not linked wadl file in v2.1

Partial-Bug: #1488144

Change-Id: Ie85acd47678f7e639528e8556fc53bde0c01a48a

dreamhost/api-site,openstack/api-site,Tesora/tesora-api-site,dreamhost/api-site,dreamhost/api-site,openstack/api-site,openstack/api-site,Tesora/tesora-api-site,Tesora/tesora-api-site,dreamhost/api-site,Tesora/tesora-api-site,Tesora/tesora-api-site,Tesora/tesora-api-site,Tesora/tesora-api-site,dreamhost/api-site,dreamhost/api-site,openstack/api-site,dreamhost/api-site