Drop private argument for include/import role

Since ansible 2.8 dropped private argument is not supported anymore:
https://github.com/ansible/ansible/issues/45038

Change-Id: Ic2d0c3de1db26b319cedab7ee1dacf997a87ff86

tasks/placement_install.yml

@@ -66,7 +66,6 @@
 - name: Run the systemd service role
   include_role:
     name: systemd_service
-    private: true
   vars:
     systemd_user_name: "{{ placement_system_user_name }}"
     systemd_group_name: "{{ placement_system_group_name }}"