lag: Support back-2-back dual mlag use case with crossed links

[jbemmel]

To date, a topology with 4 nodes implicitly assumes 2 single links between them, i.e.

links:
- lag:
   members: [ s1-s2 ]
   mlag.peergroup: True
- lag:
   members: [ s3-s4 ]
   mlag.peergroup: True  
- lag.members:
  - s1-s3
  - s2-s4

This PR adds support for crossed dual-mlag, i.e.

links:
- lag.members:
  - s1-s3
  - s1-s4
  - s2-s3
  - s2-s4

[ipspace]

Considering you had to add extra code to support the default edge-to-core MLAG wiring in most L2only fabrics, it might be time I take a look at that code :( Not going to happen soon though.