fix(docker): always use ":latest" tag in docker pull

Now that the Docker Index allows tagged builds directly from GitHub
project tags, we will version Deis components properly as
deis/controller:v0.9.0 rather than deisreleases/controller-v0.9.0.
At that point, doing a plain "docker pull deis/controller" will pull
down all tagged images when we only wanted one, so this change
prevents users from wasting more time on already slow download operations.
Being explicit about the docker image tag also facilitates our
release process.

Author: mboersma

systemd/deis-controller.service

@@ -6,7 +6,7 @@ After=deis-logger.service
 [Service]
 EnvironmentFile=/etc/environment
 TimeoutStartSec=20m
-ExecStartPre=/bin/sh -c "docker history deis/controller >/dev/null || docker pull deis/controller"
+ExecStartPre=/bin/sh -c "docker history deis/controller >/dev/null || docker pull deis/controller:latest"
 ExecStartPre=/bin/sh -c "docker inspect deis-controller >/dev/null && docker rm -f deis-controller || true"
 ExecStart=/usr/bin/docker run --name deis-controller -p 8000:8000 -e PUBLISH=8000 -e HOST=${COREOS_PRIVATE_IPV4} --volumes-from=deis-logger deis/controller
 ExecStop=/usr/bin/docker rm -f deis-controller