Code and documentation for new vas plugin option

- 2 new options added to init.pp.
- parsing of values, setting defaults.
- feature is off by default. (Backward compatability of module.)
- options used in template.
- options documented in README.md.
add spec tests for vas plugin option
Merge pull request #1 from Phil-Friderici/vas_plugin_spec_tests

add spec tests for vas plugin option
Merge pull request #13 from philippvh/vas_plugin

add support for VAS group_plugin