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/mariadb/tasks/main.yml

@@ -1,12 +1,21 @@
 ---
 - name: Add GPG key for MariaDB repo
   apt_key: id=0x177F4010FE56CA3336300305F1656F24C74CD1D8 url=https://keyserver.ubuntu.com/pks/lookup?op=get&search=0x177F4010FE56CA3336300305F1656F24C74CD1D8
+  register: add_mariadb_apt_key
   tags: mariadb, packages
 
 - name: Add MariaDB 10.3 repo
   template: src=mariadb.list.j2 dest=/etc/apt/sources.list.d/mariadb.list owner=root group=root mode=0644
+  register: add_mariadb_apt_repository
   tags: mariadb, packages
 
+- name: Update apt cache
+  apt:
+    update_cache: yes
+  when:
+    add_mariadb_apt_key is changed or
+    add_mariadb_apt_repository is changed
+
 - name: Install mariadb-server
   apt: name={{ item }} state=present update_cache=yes
   loop: