src: refactor benchmark script for 2 interface scenario

Allow for the possibility to run several tests and parametrize them.
Currently, the following characteristics can be configured:
- number of samples to take before averaging
- TCP congestion window and receive window, with the same value
- link bandwidth
- link delay
- link loss rate
- link maximum queue size
Merge branch 'master' of github.com:silviupopescu/mptcp_vpn_tun