(Fix Issue #117) Using Any other non router l3device node (i.e. server) crashes ANK

**This fixes the issue in the following way:**
1. In the ank.py, the copy_edge_attr function will no longer attempt to copy attribute from edges that are not in both graphs (actually for edges which do not have both their src and dst nodes in the destination graph, should this be even more strict and only edges in common ? ...)
2. All l3devices now have a netkit VM assigned to them, which implies that
    2a. zebra is no longer automatically started unless needed
    2b. the BGP debug flag is no longer blindly set unless bgp is enabled for the node

I am not really sure if having netkit VM's for every single l3device is really useful ... But i guess that if one was to add a l3device on the graph, he'd rather see it running ...