Smarter updating of apt index during playbook execution

We can register changes when adding repositories and keys and then
update the apt package index conditionally. This should make it be
more consistent between initial host setup and subsequent re-runs.

roles/common/tasks/tarsnap.yml

@@ -1,12 +1,21 @@
 ---
 - name: Add Tarsnap apt mirror
   template: src=tarsnap_sources.list.j2 dest=/etc/apt/sources.list.d/tarsnap.list owner=root group=root mode=0644
+  register: add_tarsnap_apt_repository
   when: ansible_architecture != 'armv7l'
 
 - name: Add GPG key for Tarsnap
   apt_key: id=0x70BD6C50E82A9D99 url=https://pkg.tarsnap.com/tarsnap-deb-packaging-key.asc state=present
+  register: add_tarsnap_apt_key
   ignore_errors: yes
 
+- name: Update apt cache
+  apt:
+    update_cache: yes
+  when:
+    add_tarsnap_apt_key is changed or
+    add_tarsnap_apt_repository is changed
+
 - name: Install tarsnap
   apt: pkg=tarsnap update_cache=yes