Add docker registry

[Xarthisius]

This PR adds docker registry to the Whole Tale stack. It includes:

• mounting both block devices into proper paths (/mnt/homes and /mnt/registry)
• generating default registry user (vars.registry_user/pass) credentials file
• passing aforementioned credentials to celery worker

In order to test:

1. deploy
2. docker login https://registry.${domain} (use vars.user/pass)
3. docker push <image>
4. docker pull <image>
5. (optionally) Try entire WT flow (adding recipe/image, building image, creating/running Tale).

[Xarthisius]

Fix for #13 should be probably included here.

[craig-willis]

Confirmed this is working up through step 4. I'm going to merge the PR, but wonder if there should be any documentation associated with this feature. Perhaps an associated PR in the design-docs repo and certainly something in admin documentation, even if just a placeholder.