Correct openrc script for zsh
Correct openrc script for zsh

Since commit 7580a0c3e37932a8fc03750d35ccd4e13e18f8c4, openrc
print a WARNING message to stdout, it will break the zsh script
in faq.rst. This patch redirect openrc output to /dev/null.

Change-Id: Iaba03634d7a234cd4d120477f91ef56d0595cdf6
Closes-Bug: #1563940

bswartz/devstack,Mahito/devstack,group-policy/devstack,richliu/devstack,group-policy/devstack,openstack-dev/devstack,bigswitch/devstack,nawawi/openstack,noironetworks/devstack,dtroyer/devstack,bswartz/devstack,nawawi/openstack,noironetworks/devstack,bigswitch/devstack,richliu/devstack,openstack-dev/devstack,dtroyer/devstack,Mahito/devstack