docker daemon: initialize the daemon pidfile early

fixes https://github.com/dotcloud/docker/issues/6973

This way there are no races on iptables, etc. before there is a discovery of existing daemon pidfile

Docker-DCO-1.1-Signed-off-by: Vincent Batts <vbatts@redhat.com> (github: vbatts)
I would use a job arg instead of the env, we generally use env for optional stuff.