This will instead discover the DC using an SRV record lookup in (r)adcli.

Requires theforeman/smart_proxy_realm_ad_plugin#19 on the proxy side

We previously enabled a workaround that breaks secureboot, as using
mkimage to build the EFI grub images results in an unsigned image. We
should only do this on 7.4, as the mac address issue is fixed in 7.5 and
later.

This parameter was removed in smart_proxy_dhcp_infobox 0.0.12.

Because we're trying to move people away from the latest repository, we
set it to configure an explicit version with a default. Because this
default would need to be updated all the time, it's a bad idea.

This patch removes the $custom_repo and makes $repo undef by default.
This means users get no repository by default and need to explicitly
choose which repository to use.

Because repo management would not be of much interest to installer
users, we move it together with $gpgcheck to the advanced section.

This is likely incorrect. We also have the type as an Optional[String]
but there's no way to unset the default. This forces a user to pick a
gateway if they have one.

This also matches the template closer to the example file.