Append to kolla user groups in bootstrap-servers

This makes the bootstrap-servers command more idempotent, since without
the append argument set the kolla user will be removed from the docker
group before being added to it again in a later task.

TrivialFix

Change-Id: Iab0f6b5e18a103e9140631ee3ebbbb48c490bc24

ansible/roles/baremetal/tasks/post-install.yml

@@ -5,6 +5,7 @@
     state: present
     group: "{{ kolla_group }}"
     groups: "sudo"
+    append: true
   become: True
   when: create_kolla_user | bool