Improve custom host attribute deletion

When you delete a custom host attributes that is still in use in one host, the deletion process does not include deleting the content in the host.mk file. Additionally, you are not aware that one host is still using this custom host attribute.

This leads to invalid data remaining in the host.mk file.

If an identical host attribute is recreated later, the value would be automatically set for the host without the user knowing about it.

Thus, it would be nice to improve this process, with a confirmation that includes a list of hosts still using the custom attributes and an option to cascade the deletion to the host.mk file.