Add tree based topology for qdr

[msimonin]

Currently we only test the complete_graph topology.
We'd like to test tree based deployment:

       +----r2
       |
r1 -------r3   
       | 
       +----r4

This corresponds to realistic scenarii where :

1. compute to conductor communication : calls on a single Target (report state).
   In this case the servers are on r1 and the clients are in r2 ... rn

2. neutron-server to agents communication push policies: fanout on a single Target.
   In this case the servers are on r2...rn and the client is in r1.