Refactor: simpify cloud_provider is defined condition

For this change, `cloud_provider` change the default value to empty
string.

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

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

@@ -166,10 +166,9 @@
 
 - name: Check cloud_provider value
   assert:
-    that: cloud_provider in ['external']
-    msg: "If set the 'cloud_provider' var must be set either to 'external'"
+    that: cloud_provider == 'external'
   when:
-    - cloud_provider is defined
+    - cloud_provider
     - not ignore_assert_errors
   tags:
     - cloud-provider