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.

[jbemmel]

Refactored lag analysis code:

"""
create_lag_interfaces -- create interfaces of type "lag" for each link marked as _virtual_lag
"""
def create_lag_interfaces(link: Box, mlag_pairs: dict, topology: Box) -> None:

  members = link.pop('lag.members',[])                        # Remove lag.members, normalized in phase I

  A: typing.Set[str] = set( [members[0].interfaces[0].node] ) # Split nodes into 'A' and 'B' groups
  B: typing.Set[str] = set( [members[0].interfaces[1].node] )
  for m in members[1:]:
    a = m.interfaces[0].node
    b = m.interfaces[1].node
    a_peer = mlag_pairs[a] if a in mlag_pairs else None
    b_peer = mlag_pairs[b] if b in mlag_pairs else None

    if a in A or a_peer in A or b in B or b_peer in B:
      A.add(a)
      B.add(b)
    elif a in B or a_peer in B or b in A or b_peer in A:
      A.add(b)
      B.add(a)
    else:
      log.error(f'Unsupported configuration of members on LAG link {link._linkname}, must consist of ' +
               'either 2, 3 or 4 different nodes connected as 1:1, 1:2 or 2:2',
                category=log.IncorrectValue,
                module='lag')
      return