docker_daemon: add pull_images option

quick and dirty poc for vendoring docker images.

I'm sure this needs more work to be merged, but I wanted to put it out there, see if you're interested in the idea and iterate.

usage:
```
plugins:
  docker_daemon:
    pull_images:
      - /usr/local/google/home/proppy/bootstrap-vz/busybox.tar.gz
      - google/docker-registry:latest
```
Why don't you just inline the pull code here?