init-template: Add needed gnupg package

Buster install from scratch fails on `puppet agent --test` run otherwise.

```
Notice: /Stage[main]/Profile::Icinga2::Apt_config/Apt::Source[icinga-stable-release]/Apt::Key[Add key: F51A91A5EE001AA5D77D53C4C6E319C334410682 from Apt::Source icinga-stable-release]/Anchor[apt_key F51A91A5EE001AA5D77D53C4C6E319C334410682 present]: Dependency Apt_key[Add key: F51A91A5EE001AA5D77D53C4C6E319C334410682 from Apt::Source icinga-stable-release] has failures: true
Warning: /Stage[main]/Profile::Icinga2::Apt_config/Apt::Source[icinga-stable-release]/Apt::Key[Add key: F51A91A5EE001AA5D77D53C4C6E319C334410682 from Apt::Source icinga-stable-release]/Anchor[apt_key F51A91A5EE001AA5D77D53C4C6E319C334410682 present]: Skipping because of failed dependencies
Warning: /Stage[main]/Profile::Icinga2::Apt_config/Apt::Source[icinga-stable-release]/Apt::Setting[list-icinga-stable-release]/File[/etc/apt/sources.list.d/icinga-stable-release.list]: Skipping because of failed dependencies
```