Refactor code for python3

[LuisBlanche]

Python 2 was abandoned , code uses some python 2 libararies like urllib2

[nmgeek]

$ docker-push-ssh
Traceback (most recent call last):
  File "/usr/local/bin/docker-push-ssh", line 3, in <module>
    from docker_push_ssh import cli
  File "/usr/local/lib/python3.6/dist-packages/docker_push_ssh/cli.py", line 19, in <module>
    import urllib2
ModuleNotFoundError: No module named 'urllib2'

[superhawk610]

For anybody else who ends up here, until Luis's PR (#15) is merged, you can install directly from Github

$ pip install --upgrade --user git+https://github.com/LuisBlanche/docker-push-ssh@python3