Daemonize jenkins-log-pusher.

* modules/openstack_project/files/logstash/log-pusher.py: Semi properly
daemon the log pusher process by default. Close open file descriptors,
fork into background, detach from terminal, redirect std* to /dev/null,
change the umask to 0, change working dir to '/', and lock a PID file.

* modules/openstack_project/files/logstash/jenkins-log-pusher.init:
Update start-stop-daemon commands to use the presence of a PID file and
no longer background with start-stop-daemon.

Change-Id: I4dcdd48478fa7d27745a3075a6942838e9df20ee
Reviewed-on: https://review.openstack.org/28449
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Approved: James E. Blair <corvus@inaugust.com>
Reviewed-by: James E. Blair <corvus@inaugust.com>
Tested-by: Jenkins