"once instance" -> "one instance"

"...run on all controller nodes, ensuring at least *one* instance will be
available in case of node failure."

It was correct (ie "one") for neutron's row in table, but had to be corrected
(ie "once" -> "one") for horizon, keystone, glance, nova, & cinder.

Change-Id: I99b6d479ee0ee3ea9710fc0834ebcb414389f1f1