bench_env: Initial version of the benchmark environment setup

A set of test cases with the goal of measuring easily the peformance
between snabbswitch and virtual machines through vhost_user.

Included three main test cases:
- guest to guest:
    guest0 -> snabb nfv0 -> NIC0 = NIC1 <- snabb nfv1 <- guest1
- guest to host:
    guest0 -> snabb nfv0 -> NIC0 = NIC1 <- host
- guest to loadgen
    guest -> snabb nfv -> NIC_0 = NIC_1 <- snabb loadgen

Some helper scripts are also included:
- NIC rebinding
- guest to guest through vhost-tap

Signed-off-by: Alexander Spyridakis <a.spyridakis@virtualopensystems.com>