docker: only use systemd to manage containers

docker: only use systemd to manage containers

Prior to this patch we had several ways to runs containers, we could use
ansible's docker module on some distro and on containers distros we were
using systemd. We strongly believe threating containers as services with
systemd is the right approach so this patch generalizes to all the
distros. These days most of the distros are running systemd so it's fair
assumption.

Signed-off-by: Sébastien Han <seb@redhat.com>