Add ability to subfilter in sls files
Add ability to subfilter in sls files

This will recurse through the sls when matching the minion.

It will only recurse on the special key: subfilter

Why subfilters? There a few benefits. Notice in the example below
the top most filter is environment specific. All the sub filters can
be copied, as is, beneath any filter. No more writing '*dev*web*front'
anymore.

For further documentation on why this was added:
Please see issue saltstack/salt#9979

Here is an example sls:

```yaml
dev:
  '*dev*':
    - sqlite
    - subfilter:
        '*webserver*':
          - nginx
prod:
  '*prod*':
    - stunnle
    - subfilter:
        '*webserver*':
          - newrelic-agent
          - subfilter:
              '*front':
                - haproxy
              '*back':
                - nginx
                - statsd
```

saltstack/salt,saltstack/salt,saltstack/salt,saltstack/salt,saltstack/salt