Fix role gate tests for Ansible 2.3

'ansible_become' defined as a host_var or in the inventory for a host
has a higher precedence than 'become' defined within a playbook.

In change Id5b76a87809f03951c954fc3d752419a673403f7, 'become' was
defined explicitly for each play running against localhost so
'ansible_become' should be removed from its host_vars.

Change-Id: Ieb55a10e8148b0eb122caccf92702d24ff1a1d0b
Cc: Jimmy McCrory <jimmy.mccrory@gmail.com>

tests/inventory

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

tests/test.yml

@@ -16,6 +16,7 @@
 - name: Playbook for role testing
   hosts: localhost
   connection: local
+  become: true
   pre_tasks:
     - name: Show host facts
       debug: