systemd: EnvironmentFile for additional options

This commit allows the systemd unitfile to use additional options
for docker daemon.

	modified:   contrib/init/systemd/docker.service
	new file:   contrib/init/systemd/docker.sysconfig

Docker-DCO-1.1-Signed-off-by: Lokesh Mandvekar <lsm5@fedoraproject.org> (github: lsm5)