(#3299) add the file attributes owner, group and mode to nagios types

It's desirable to make it possible to write manifests such as

nagios_host { "hostname":
  ... # nagios properties
  group => "nagios",
  mode  => 640,
}

instead of the workaround of

nagios_host { "hostname":
  target => "/etc/nagios/hostname.cfg",
  ...
}
file { "/etc/nagios/hostname.cfg": ... }

To make this possible in a simple fashion, just add the file properties
owner, group and mode to each nagios type as parameters. If either of them
is specified, the nagios resource generates an additional file resource
and passes the parameters as should-values of the respective properties.
(#3299) simple unit test for the new nagios parameters

For now, only test for the presence of the parameters. Their function should
be tested in an integration spec.
(#3299) integration spec for the nagios type

This is a very rough draft for a spec test wrt. the "mode" parameter.
Testing group and owner is difficult since unprivileged rspec users
cannot use chown or chgrp.

This is open for discussion and should be revised before merging.