Fix role gate tests for Ansible 2.3

Add 'become' and root 'user' options to the playbook instead of using
'ansible_become' through an inventory variable.

Change-Id: Id51a7882540a68e53ef00a26e910214a6df6ec9f

tests/inventory

@@ -1 +1,2 @@
-localhost ansible_connection=local ansible_become=True
+[all]
+localhost

tests/test.yml

@@ -16,5 +16,6 @@
 - name: Playbook for role testing
   hosts: localhost
   connection: local
+  become: true
   tasks:
     - debug: msg="Testing is still to be implemented"