Bench

Updates the bench scripts environment to be used in automatic testing scenarios.
On chur to use it one needs to check out the tree and then just run the script:

>sudo ./src/scripts/bench_env/loadgen-snabb-nic-guest.sh
>...
>11      sent on vm.tx -> nic.rx
>10,000,173      sent on nic.tx -> vm.rx
>
>Processed 10.0 million packets in 13.41 seconds
>Rate(Mpps):     0.745
>Waiting QEMU processes to terminate...
>Finished.

The chur setup consists of the following:
 - /etc/bech_conf.sh
 - /opt/bench
   - /opt/bench/bzImage - the Ubuntu 14.04 kernel
   - /opt/bench/qemu/obj/x86_64-softmmu/qemu-system-x86_64 - QEMU with vhost-user-v10 patches
   - /opt/bench/ubuntu-trusty.img0 /opt/bench/ubuntu-trusty.img1 - Ubuntu 14.04 rootfs images

One can override the /etc/bench_conf.sh by copying it in his $HOME and edit it.

By default the snabb executable path is evaluated as a relative to the script execution path.
This behavior can be override in 2 ways:
 - specify SNABB=\<path to snabb\> in bench_conf.sh
 - specify SNABB=\<path to snabb\> as an env variable e.g.:

>sudo SNABB=\<path to snabb\> ./src/scripts/bench_env/loadgen-snabb-nic-guest.sh