"instance_usage_audit Hhour"->"instance_usage_audit_period hour"

In Point-2, change the command "openstack-config --set /etc/
nova/nova.conf DEFAULT instance_usage_audit Hhour" to
"openstack-config --set /etc/nova/nova.conf DEFAULT
instance_usage_audit_period hour"

The description about editing the
/etc/nova/nova.conf [DEFAULT] section has already been added by
someone else.

Change-Id: I978790c6a6c8d9a122ac972c6d3758a694645290
backport: havana
Closes-Bug: #1249779
(cherry picked from commit 69a73be10399e47b75d594f541b94ac433603201)
Merge ""instance_usage_audit Hhour"->"instance_usage_audit_period hour"" into stable/havana