Fix check external_openstack_tenant_name value (#6270)

We need to specify either external_openstack_tenant_name or
external_openstack_tenant_id. Those values were checked by seeing they
are defined or they have actual values separately.
However those values are always defined because of the following code
of openstack/defaults/main.yml:

external_openstack_tenant_id: "{{ lookup('env','OS_TENANT_ID')| default(lookup('env','OS_PROJECT_ID'),true) }}"
external_openstack_tenant_name: "{{ lookup('env','OS_TENANT_NAME')| default(lookup('env','OS_PROJECT_NAME'),true) }}"

So even if not specifying both values, those checks could not detect
the misconfiguration. This fixes this to detect the misconfiguration.