Manifest to install Opsmatic agent

The way to use the new class to install Opsmatic agent is:

    class { 'opsmatic::agent':
        token => $token,
        credentials => $debian_credentials,
    }

Don't provide a blank default here, instead make sure that the credentials actually HAVE to be submitted:

    class opsmatic::debian-private (
      $credentials,
    ) {