Add IGP information to loopback interface

[ipspace]

We cannot use netlab_interfaces in most IGP configuration templates because there's no IGP information on the loopback interface. OSPF is an exception -- ospf.area is set on node.loopback.

The minimum IGP information we need is an empty dictionary, more information can be included from devices.device.features.igp.loopback dictionary.

This issue should add IGP information for OSPFv2/OSPFv3, IS-IS, EIGRP, and RIPv2 as well as reference implementation for Arista EOS and Cisco IOSv.