change OpenVPN tunnel parameters

The following changes have been made to OpenVPN tunnels:
- no encryption
- no HMAC
- increase tunnel MTU such that larger buffers are passed to
OpenVPN; even though we are no longer performing encryption,
the buffers still must pass to user-space
- deactivate OpenVPN internal fragmentation
- deactivate OpenVPN TCP Maximum Segment Size option

The values 10000 for UDP and 50000 for TCP have been determined
experimentally and have shown to provide the best throughput in
a normal scenario (no tc, no multipath). However, is is worth
remarking that iperf TCP over OpenVPN TCP will exhibit quite a
random behavior and the throughput might vary greatly.
update plot script