[validate-tempest] use 'become' instead of 'sudo'

- This patch avoids deprecations warning about using sudo/sudo_user
  instead of become/become_user.
- Adds new line at the end of roles/../tasks/undercloud-config.yaml file.

Change-Id: I6c7419bafaec93c63454ed269fde3872ae15dede
Signed-off-by: Gael Chamoulaud <gchamoul@redhat.com>

roles/validate-tempest/tasks/undercloud-config.yml

@@ -2,7 +2,7 @@
 # tasks file for ansible-role-tripleo-tempest-undercloud
 
 - name: Add fake driver to Ironic config
-  sudo: yes
+  become: yes
   lineinfile:
     backup: true
     backrefs: true
@@ -11,7 +11,7 @@
     line: '\1,fake'
 
 - name: Restart the Ironic conductor service
-  sudo: yes
+  become: yes
   service: name=openstack-ironic-conductor state=restarted
 
 - name: Ensure the fake driver is enabled
@@ -24,4 +24,4 @@
 - name: Copy tempest input file to undercloud
   copy:
     src: "tempest-undercloud-config.conf"
-    dest: "/home/stack/{{ tempest_deployer_input_file }}"
+    dest: "/home/stack/{{ tempest_deployer_input_file }}"