Update initial form for IP trunk creation

The input form for creating a new IP trunk is now a three-step process, where side A and B are steps 2 and 3.

• It is no longer possible to select the same device for both sides.
• The min number of links is enforced as a required amount of inputs.

@simone.spinelli the input step enforces IPTRUNK_MINUMUM_LINKS to be the exact amount of links that should be added. Is this intended behaviour?