Fix BIRD config generation

[pierky]

Before of this:

$ ./bgperf.py bench -t bird
run bird
remove container: bird
Traceback (most recent call last):
  File "./bgperf.py", line 406, in <module>
    args.func(args)
  File "./bgperf.py", line 178, in bench
    target.run(conf, brname)
  File "/home/pierky/bgperf/bird.py", line 165, in run
    self.write_config(conf)
  File "/home/pierky/bgperf/bird.py", line 155, in write_config
    for n in sorted(conf['tester'].values() + [conf['monitor']], key=lambda n: n['as']):
KeyError: 'tester'

[ishidawataru]

thanks!