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.
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