Module `icinga2_mysql': require `Icinga2::Config[...]' rather than `File[...]'

refs #6842
icinga2::feature: don't require Package[icinga2]

refs #6842
icinga2::config: hardcode '/etc/icinga2' once less

refs #6842
icinga2::feature: replace `exec' with `file'

refs #6842
Make `icinga2::feature' responsible for `icinga2::config'

refs #6842