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: I3b56a8c2df670423a2cb4e473379028e23bb01bd

tests/host_vars/localhost.yml

@@ -14,4 +14,3 @@
 # limitations under the License.
 
 ansible_python_interpreter: "/usr/bin/python2"
-ansible_become: True

tests/test-install-previous-swift.yml

@@ -16,6 +16,7 @@
 - name: Playbook for deploying swift
   hosts: "{{ swift_groups | default('swift_all:swift_remote_all') }}"
   user: root
+  become: true
   gather_facts: true
   roles:
     - role: "os_previous_swift"