Add a way for users to manually specify project in addition to image family

[lbajolet-hashicorp]

Follow-up to #191/#192, this issue/PR combo exists because the list of projects to get an image from when an image family is specified in a template is hardcoded in the plugin.

This doesn't scale, and implies that users wanting to use an image from one of those plugins should open an issue and us maintainers have to add the project to the list, then release a new version of the plugin.

Instead, we should probably add a new attribute so users that want an image from a specific project/family have the option to specify it manually, and we don't need to release a new version of the plugin each time.

[twistedpair]

I'm also impacted by this.

The project deeplearning-platform-release is used by GCP Deep Learning VM Images, but is missing in the hardcoded list.

Simply being able to pass a source_image_project value through would be helpful.