Add URL handling for hook return string

[akarasulu]

Right now hooks return a file path. We should allow returning a URL and optionally switch to pulling down the artifact as a provisioning step inside the guest. This way the guest can pull down the snap or management Debian package and install it. This switch should occur automatically.

We will make this work with the CDN also to make sure caching is taken advantage of. There are some issues which this depends on:

subutai-io/cdn#171

And the issue to change devops to push the snap into Gorjun after building.