Fixes #8005 - Convert allowed NIC types to strings

@tstrachota could you please check? If you use zeus you can reproduce the autoloading issue by running
```
zeus test test/functional/unattended_controller_test.rb
```
that would be nice, but we should change it also in display actions, @tstrachota can we add some mapping easily? e.g. Nic::Managed is "Interface" in WebUI