Refactor: simpify external_cloud_provider is defined

For this change, `external_cloud_provider` set default value to empty
string.

Signed-off-by: ChengHao Yang <17496418+tico88612@users.noreply.github.com>

roles/kubernetes/preinstall/tasks/0040-verify-settings.yml

@@ -174,6 +174,16 @@
     - cloud-provider
     - facts
 
+- name: Check external_cloud_provider value
+  assert:
+    that: external_cloud_provider in ['hcloud', 'huaweicloud', 'oci', 'openstack', 'vsphere']
+  when:
+    - cloud_provider == 'external'
+    - not ignore_assert_errors
+  tags:
+    - cloud-provider
+    - facts
+
 - name: "Check that kube_service_addresses is a network range"
   assert:
     that: