[admin-guide] Update the "nova" command to "openstack"
[admin-guide] Update the "nova" command to "openstack"

Change-Id: I937e77033bc83b09f3d5c9d23ee90cdf25b36646

openstack/openstack-manuals,openstack/openstack-manuals,openstack/openstack-manuals,openstack/openstack-manuals