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: Ic739c776dfb050e18828f2908f9fc1acc02bd90c

tests/host_vars/localhost.yml

@@ -18,4 +18,3 @@ bridges:
     ip_addr: "10.1.0.1"
 
 ansible_python_interpreter: "/usr/bin/python2"
-ansible_become: True

tests/test-install-previous-neutron.yml

@@ -18,6 +18,7 @@
 - name: Make /lib/modules accessible on neutron_agent containers
   hosts: neutron_agent
   user: root
+  become: true
   gather_facts: true
   tasks:
     - name: Use the unconfined aa profile