openstack can now use --edge-prebuilt

Also: update to new endpoints for prebuilt stemcells/amis for AWS
Download stemcell in own stage; use etag in name for edge-prebuilt
Note to @drnic about how to clean up CI AWS env
The ? in sed expr didn't fix issue in filename [ci skip]
test prebuilt in ci
Merge pull request #178 from StarkAndWayne/prebuilt-openstack

Prebuilt openstack & other aws regions