Add varstack ext_pillar

This PR adds an external pillar using the Varstack system. Varstack ist very similar to the puppet related hiera project and allow to merge multiple sets of data into a single combined set of variables.

In order to use this ext_pillar module Varstack has to be installed from this location:
https://github.com/conversis/varstack

To activate the pillar the ext_pillar section in the saltstack master configuration has to look like this:
```
...
ext_pillar:
  - varstack: /etc/varstack.yaml
...
```

More information about Varstack itself can be found here:
https://github.com/conversis/varstack/blob/master/README.md