GitLab

* Add ability to manage icinga2.conf

* Enable management of constants.conf

closes #9

* Update README.md

* Cleanup test.yml

* Refactor i2_const_* vars to i2_constants dict

Using the combine filter, we can remove the `i2_const_*` variables and
have a generic `i2_constants` dictionary from which we can read the
user-specified constants (`i2_custom_constants`) and our defaults
(`i2_default_constants`). We lose the comments in the process however.

* Copy constants file before overriding OS defaults