Add Chef recipe for Mininet

Now Mininet is automatically installed in the VM in
`/home/vagrant/test-env/mininet/` directory. It greatly facilitates
setting up testing environments for LINC-Switch.
(Motivated by FlowForwarding/LINC-Switch#167)
Merge pull request #2 from mentels/mininet

Motivated by FlowForwarding/LINC-Switch#167