Allow override of the repo filename

Apt cannot have 2 mirrors with the same content in 2 different files.
This commit makes possible to decide the filename for the repo to
avoid possible clashes.

Change-Id: I8be9b49e40b755cb90928067c60fe8a5d9c5ae0b

releasenotes/notes/apt-source-filenamed-4b0f490c2bd97c19.yaml

@@ -0,0 +1,6 @@
+---
+features:
+  - The filename of the apt source for the ubuntu
+    cloud archive used in ceph client can now be
+    defined by giving a filename in the uca part
+    of the dict ``ceph_apt_repos``.

tasks/ceph_preinstall_apt.yml

@@ -71,6 +71,7 @@
   apt_repository:
     repo: "{{ ceph_apt_repos[ceph_pkg_source].repo }}"
     state: "{{ ceph_apt_repos[ceph_pkg_source].state }}"
+    filename: "{{ ceph_apt_repos[ceph_pkg_source].filename | default(omit) }}"
     update_cache: yes
   register: add_repos
   until: add_repos|success