Add push/pull alternative to save/load test images

[dmcgowan]

Allow loading images into the test container using pull rather than load. While load is fast, saving the image the images in the test image container could be a hindrance to running golem in a distributed environment. Doing a docker push/pull can be just as efficient as save/load but without the overhead of repeatedly saving each image to build the test image.

The best way to accomplish this is by allowing the golem binary to take in a repository namespace as an argument. Each test image will have create an image repository under that namespace and push each image into the repository using a different tag. This will be able to take advantage of cross-repository push and already existing layers to make subsequent test runs more efficient. The test images should be built using the tag mapping of target -> test-repository@image-digest. This will ensure any change in namespace repository re-issues pushes and changes to tags do not cause changes to test images.