Currently, if you run the os-nova-install.yml playbook with the
nova-config or nova-install tags, the run will fail as nova_virt_type
is not defined anywhere. This commit updates os_nova/tasks/main.yml by
adding the always tag so that nova_virt_detect.yml is always included.
Change-Id: Id0fdd12aecc63c066da985fbb058cb85926b2ff3