Remove project_id from alternate image link path

Removes the project id from the alternate link path
generated for the v3 API os-images extension.

Also fixes issue with os.path.join being used to generate
urls which will break on Windows.

Partially implements blueprint nova-v3-api

Change-Id: Icc34359cc935920e2d4780849d9310c4961c8f74
Merge "Remove project_id from alternate image link path"