OpenStack synchronizer: add ability to download file via URL

Add the ability to add files by URL to OpenStack.

Currently the build system is responsible for downloading files from URLs, copying them to the head node, and mounting them into the OpenStack synchronizer.  We want to streamline this process for the Helm-based install where we don't have a build system.  

We'll allow the "path" field of the image to have either a file path or a URL.  If it's a URL, the synchronizer will download the file and then load it into Glance.