Added two new environment variables: DOCKER_SSL_VERSION and DOCKER_ASSERT_HOSTNAME

These two environment variables correspond to the ssl_version and assert_hostname arguments of the docker.utils.kwargs_from_env function.
This is needed for configuring one Docker host based purely on environment variables.

For example, you can inventorize a boot2docker instance this way:

    eval "$(boot2docker shellinit)"
    export DOCKER_ASSERT_HOSTNAME="boot2docker"
    ./docker.py --list