Allow for config settings to be overridden by a `Saltfile`

The order of configuration handling is the following:
1. If the user passes an option to the parser, that option rules.
2. If no option is passed to the parser and that option is set a
`Saltfile`, that option rules.
3. If none of the above and such option is set in the cofiguration file,
that value rules
4. As a last resort the default value rules

The format of the `Saltfile` is:

```yaml
<salt-cli-tool>:
  <confi-key>: <config-value>

salt-key:
  ssh_max_procs: 50
```
Make sure the user knows a Saltfile will be loaded.