Refactor respository fetcher
Refactor respository fetcher

Factored out "getting the image ID" from the enormous fetcher.

Signed-off-by: Gareth Smith <97ec7f0e37ed298c6c7d61e7dba0820f215e55e4@pivotal.io>

cloudfoundry-incubator/garden-linux,cloudfoundry-incubator/garden-linux,cloudfoundry-incubator/garden-linux