Possible regression in multi-domain breakdown

[sajith]

It seems that the original connection request that we've been testing with used to be able to produce a breakdown that spans three domains, but now pce is unable to find a solution for the same connection request.

This is a follow up to issue #112 and PR #113. It would be useful to get PR #113 merged first, to prevent it from growing anymore. I ended up writing a test to investigate this issue in that PR.

[YufengXin]

The original topology json model is first converted to a networkx graph model, before forming the PCE optimization model. It would be useful to debug on the networkx graph model first, eg, generating the graph in a image, mapping the connection request to this graph to see if a path exist or not, etc.

[YufengXin]

Added a comment in SDX-controller:

atlanticwave-sdx/sdx-controller#145