Implement user-api

This implements the blueprint user-api for version 1 of the API.

The ACL checks now don't return access denied on non-admin users.

All requests without a <project> in the URL limit their scope to the tenant
contained into X-Tenant-Id.

All request with a <project> in the URL returns 404 if the <project>
specified is different than the one from X-Tenant-Id.

Change-Id: If1ec55fa491ea5de30036ce7ed75d0f28e925457
Signed-off-by: Julien Danjou <julien@danjou.info>
Merge "Implement user-api"