Force the uca filename

Defining staticly the UCA filename allows us to know in advance
which file to copy from host in the lxc_host role.

We remove here previously defined or default UCA filenames for
the same branch.

Change-Id: If6e3fe6b6c24570c34fe7bab358aed59340f379c

releasenotes/notes/static_uca_filename-849a6f491acae9c5.yaml

@@ -0,0 +1,11 @@
+---
+upgrade:
+  - |
+    Any user that is coming from Pike or below on Ubuntu should modify
+    its ``user_external_repos_list``, switching its ubuntu cloud archive
+    repository from ``state: present`` to ``state: absent``.
+    From now on, UCA will be defined with the filename ``uca``. If the deployer
+    wants to use its mirror, he can still override the variable ``uca_repo``
+    to point to its mirror. Alternatively, the deployer can completely define
+    which repos to add and remove, ignoring our defaults, by overriding
+    ``openstack_hosts_package_repos``.

vars/ubuntu-16.04.yml

@@ -89,6 +89,16 @@ _package_list:
     state: "{{ openstack_hosts_package_state }}"
 
 _package_repos:
+  # For convention over configuration, this removes any UCA configuration with
+  # default filename, or user defined filename.
+  # Deployers can/should use this facility to remove any previously user defined
+  # repositories.
+  # TODO(evrardjp): Add a similar 3 liner when we bump the uca_repo to Queens,
+  # to have both queens and Pike removed.
+  - repo: "{{ uca_repo }}"
+    state: absent
+    filename: "{{ uca_apt_source_list_filename | default('ubuntu_cloud_archive_canonical_com_ubuntu') }}"
+  # Always use UCA filename
   - repo: "{{ uca_repo }}"
     state: present
-    filename: "{{ uca_apt_source_list_filename | default(omit) }}"
+    filename: "uca"