Add controller's scenario for sample Mininet topology

This scenario setup two switches for the topology defined in
MININET_ROOT/custom/topo-2sw-host.py.
(Motivated by FlowForwarding/LINC-Switch#240).
Update LINC/Mininet tutorial after adding support for multiple instances

Now LINC can be run in Mininet topologies with more than one switch.
(Motivated by #240).