fixes #6329 - toggle label content view/puppet environment, toggle show/hide installation media on new host form

an easier way might be:

```ruby
validates :medium_id, :presence => false, :if => Proc.new { |host| host.skip_media_validation? }
```

in foreman
```ruby
def skip_media_validation
  content_source_id.present?
end
```

and in katello, override that method?