[user-guide] Update the "nova host-list" to "openstack host list"
[user-guide] Update the "nova host-list" to "openstack host list"

Change-Id: I125409449cf211e8d48e7b71743a1ad5abafba0d
Implements: blueprint use-openstack-command

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