Create test images via Glance instead of Nova

When possible, create blank test images in Glance instead of relying on
snapshotting, which may not be supported on the hypervisors under test.

Change-Id: I56f79b0ae718b2dcc43853bed3a792f15eccc313
Updated from global requirements

Change-Id: I1c05c375be86dffd8b4fadd536c170df3c2cd3ed
Merge "Create test images via Glance instead of Nova"
Merge branch 'master' of https://github.com/openstack/tempest