Add basic benchmark

Benchmark shows performance penalty rootwrap introduces compared to one of
plain sudo.

To run benchmark, issue: tox -e benchmark

Example output:

Running 'ip a':
        method          :    min       avg       max       dev
                   ip a :   4.185ms   4.570ms   5.657ms 187.705us
              sudo ip a :  13.564ms  14.437ms  28.452ms   1.470ms
sudo rootwrap conf ip a : 148.839ms 192.424ms 254.043ms  19.219ms
Running 'ip netns exec bench_ns ip a':
                    method                     :    min       avg       max       dev
              sudo ip netns exec bench_ns ip a : 109.772ms 151.627ms 209.943ms  22.991ms
sudo rootwrap conf ip netns exec bench_ns ip a : 289.345ms 345.471ms 463.807ms  32.873ms

Change-Id: Id8e41be6602fa8dcff48a8a4ba44d35dd3043731