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/network_plugin/calico/tasks/pre.yml

@@ -29,7 +29,7 @@
   register: calico_kubelet_name
   delegate_to: "{{ groups['kube_control_plane'][0] }}"
   when:
-  - "cloud_provider is defined"
+  - cloud_provider
 
 - name: Calico | Gather os specific variables
   include_vars: "{{ item }}"