Allow fallback to Newton default filename

If the deployer decides to override the complete apt repository
git, it would be best to have the possibility to NOT define a
filename, this way it would be the same behavior as in N.

This also allows a combined backport of
https://review.openstack.org/#/c/417362/
to fix bug 1653947 cleanly in Newton.

Change-Id: I00b924275cd9350a93b48f4d886919b597392049

tasks/galera_install_apt.yml

@@ -73,7 +73,7 @@
 - name: Add galera repo
   apt_repository:
     repo: "{{ galera_repo.repo }}"
-    filename: "{{ galera_repo.filename }}"
+    filename: "{{ galera_repo.filename | default(omit) }}"
     state: "{{ galera_repo.state }}"
   register: add_repos
   until: add_repos|success
@@ -85,7 +85,7 @@
 - name: Add percona repo
   apt_repository:
     repo: "{{ galera_percona_xtrabackup_repo.repo }}"
-    filename: "{{ galera_percona_xtrabackup_repo.filename }}"
+    filename: "{{ galera_percona_xtrabackup_repo.filename | default(omit) }}"
     state: "{{ use_percona_upstream | bool | ternary('present','absent') }}"
   register: add_repos
   until: add_repos|success