tests_vlan_mtu: Properly set the network provider

Set the network_provider as fact to ensure that it is also set in the
imported playbook.

tests/tests_vlan_mtu_initscripts.yml

@@ -1,12 +1,13 @@
 ---
-# empty playbook to gather facts for import_playbook when clause
+# set network provider and gather facts
 - hosts: all
-  vars:
-    network_provider: initscripts
+  tasks:
+    - name: Set network provider to 'initscripts'
+      set_fact:
+        network_provider: initscripts
 
 # workaround for: https://github.com/ansible/ansible/issues/27973
 # There is no way in Ansible to abort a playbook hosts with specific OS
 # releases Therefore we include the playbook with the tests only if the hosts
 # would support it.
-# The test requires NetworkManager, therefore it cannot run on RHEL 6 or CentOS 6.
 - import_playbook: playbooks/tests_vlan_mtu.yml

tests/tests_vlan_mtu_nm.yml

@@ -1,8 +1,10 @@
 ---
-# empty playbook to gather facts for import_playbook when clause
+# set network provider and gather facts
 - hosts: all
-  vars:
-    network_provider: nm
+  tasks:
+    - name: Set network provider to 'nm'
+      set_fact:
+        network_provider: nm
 
 # workaround for: https://github.com/ansible/ansible/issues/27973
 # There is no way in Ansible to abort a playbook hosts with specific OS