Remote image deploy

[Szubie]

This PR enables use of image stored in remote LXD servers in Bravefiles. These remote images can be used as "Base" images during build or as "source" images during container deployment. In addition, during the build phase it is now possible to push the resulting image into a remote LXD server.

An example Bravefile using these features:

image: store:test-image/1.0
base:
  image: store:alpine/edge

service:
  name: deploy:python-api

During build:
The "store" remote provides the base image for the build phase ("alpine/edge"). The resulting image "test-image/1.0" is built and then pushed into the "store" remote server.

During deploy:
The "store" remote holds the source image "test-image/1.0" to be deployed as a container. If bravetools does not have a local copy of the image already it will import it from the "store" remote. Then the resulting container will be pushed to the "deploy" remote.

Bravetools will always make a local copy of images it imports when possible, whether during build or if deploying a remotely stored image. Local images will be prioritized over remotely stored images if present; this last point may be debated - perhaps it's better to do the opposite and favour remote images by default instead? Or perhaps some validation could be done like checking fingerprints.

The mechanism used to import these images is the same as brave base, meaning a container will be spun up and turned into an image and exported as a single .tar.gz file.