Add support to add environment variables to Docker daemon

[haxorof]

If you want to for example configure proxy settings for Docker this shall be put as environment variables to the process which is described here.

In this role it now exists an environment file referenced by a systemd drop-in which can be extended to add additional environment variables like HTTPS_PROXY etc.

https://github.com/haxorof/ansible-role-docker-ce/blob/master/templates/docker-envs.j2