docker: fix parsing of docker __version__ string

If `docker.__version__` contains non-digit characters, such as:

    >>> import docker
    >>> docker.__version__
    '1.4.0-dev'

Then `get_docker_py_versioninfo` will fail with:

    ValueError: invalid literal for int() with base 10: '0-de'

This patch corrects the parsing of the version string so that
`get_docker_py_versioninfo` in this example would return:

    (1, 4, 0, '-dev')

cloud/docker/docker.py

@@ -481,6 +481,7 @@ def get_docker_py_versioninfo():
                     if not char.isdigit():
                         nondigit = part[idx:]
                         digit = part[:idx]
+                        break
                 if digit:
                     version.append(int(digit))
                 if nondigit: