[TRAIN ONLY] Update tripleo-ansible before uc update/upgrade.

Currently in the downstream product we enforce that rule with a
documentation modification.  But it's not the prefered way and
shouldn't be the rule.

So we make that *temporary* patch in train only until downstream
catch up with the reviews[1][2] proposed which doens't involve user changing
his procedure.

[1] https://review.opendev.org/718683
[2] https://review.opendev.org/717719

Change-Id: I807d1c6dceb332480279a36f205b96bbea212024
Related-Bug: #1869776

tasks/common/undercloud_prerequisites.yaml

@@ -12,4 +12,5 @@
       - "openstack-tripleo-common"
       - "openstack-tripleo-heat-templates"
       - "openstack-tripleo-validations"
+      - "tripleo-ansible"
     state: latest