Added generic icinga2::conf define @7273

Signed-off-by: Nick Chappell <nick@intronic.org>

Merged in from: https://github.com/Icinga/puppet-icinga2/pull/12

Refs #7273: https://dev.icinga.org/issues/7273
README additions for the icinga2::conf type.
Use the operatingsystemmajrelease fact for CentOS and Ubuntu in metadata.json.
Correct a typo with the operatingsystemmajrelease fact name.
Merge pull request #1 from Icinga/master

Upgrading...