icinga2_host: make use of templates and template vars (#6286)
* icinga2_host: make use of templates, append vars instead of replacing all vars array.
* Initialize `template` variable. Add changelog fragment.
* Update changelogs/fragments/6286-icinga2_host-template-and-template-vars.yml
Co-authored-by: Felix Fontein <felix@fontein.de>
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Co-authored-by: Felix Fontein <felix@fontein.de>
(cherry picked from commit 76dd465e08)
Co-authored-by: yoannlr <32494673+yoannlr@users.noreply.github.com>
