add icinga2::feature definition

I've left this onlyif commented out to cause a failure on a missing feature. Uncommented will silently fail to do anything since the onlyif will not match on a missing file.

Example of the failure:

```
==> centos7-01: Error: /Stage[main]/Main/Node[centos7-01.test.vagrant]/Icinga2::Server::Feature[commandtt]/Exec[icinga2_enable_feature_commandtt]/returns: change from notrun to 0 failed: icinga2-enable-feature commandtt returned 1 instead of one of [0]
```