Does not respect `DOCKER_CONFIG` environment variable

[pmeinhardt]

The pierone CLI always writes to ~/.docker/config.json and does not respect a different configuration path indicated by the DOCKER_CONFIG environment variable:

pierone-cli/pierone/api.py

Line 162 in dc7a732

path = os.path.expanduser('~/.docker/config.json')

[hjacobs]

Do you mind doing a PR with a fix?

[pmeinhardt]

Here you go: #90 ✌️