fixes #4589 - adds conditional output field to show network interfaces

Fixes http://projects.theforeman.org/issues/4589

I discussed with @tstrachota to make those lists additional as compute resources do not have them. 

I split up managed- and BMC-interfaces as BMC interfaces have the additional username/password fields. I didn't want to make them `:hide_blank => true` as a blank username/password could be a valid option. 
I found no way to properly factor this into one basic call + the additional options in the BMC collection. Any ideas?