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
```