Rename roles

Allow roles to be renamed without having to manually stop the old
containers.

If you have config like:

```
servers:
  jobs:
    hosts:
      - vm3
```

And you want to rename `jobs` to `workers`, you can do:

```
servers:
  workers:
    previously:
      - jobs
    hosts:
      - vm3
```

And the deployment will take care of stopping the old "jobs" containers.

Once deployed you can remove the `previously` key.

test/integration/docker/deployer/rename_roles.sh

@@ -0,0 +1,3 @@
+#!/bin/bash
+
+cd $1 && cp -f config/deploy_renamed_roles.yml config/deploy.yml && git commit -am 'Rename roles'