fix handling of 'None' RepoTags
fix handling of 'None' RepoTags

fix the folloing bug:

fujita@ubuntu:~/git/bgperf$ ./bgperf.py prepare
Traceback (most recent call last):
  File "./bgperf.py", line 362, in <module>
    args.func(args)
  File "./bgperf.py", line 73, in prepare
    ExaBGP.build_image(args.force, nocache=args.no_cache)
  File "/home/fujita/git/bgperf/exabgp.py", line 33, in build_image
    super(ExaBGP, cls).build_image(force, tag, nocache)
  File "/home/fujita/git/bgperf/base.py", line 111, in build_image
    if force or not img_exists(tag):
  File "/home/fujita/git/bgperf/base.py", line 32, in img_exists
    return name in [ctn['RepoTags'][0].split(':')[0] for ctn in dckr.images()]
TypeError: 'NoneType' object has no attribute '__getitem__'

Signed-off-by: FUJITA Tomonori <93dac1fe9c4b2a3957982200319981492ad4976e@lab.ntt.co.jp>

osrg/bgperf