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/nginx/tasks/main.yml

@@ -1,12 +1,21 @@
 ---
 - name: Add nginx.org apt signing key
   apt_key: id=0x573BFD6B3D8FBC641079A6ABABF5BD827BD9BF62 url=https://nginx.org/keys/nginx_signing.key state=present
+  register: add_nginx_apt_key
   tags: nginx, packages
 
 - name: Add nginx.org repo
   template: src=nginx_org_sources.list.j2 dest=/etc/apt/sources.list.d/nginx_org_sources.list owner=root group=root mode=0644
+  register: add_nginx_apt_repository
   tags: nginx, packages
 
+- name: Update apt cache
+  apt:
+    update_cache: yes
+  when:
+    add_nginx_apt_key is changed or
+    add_nginx_apt_repository is changed
+
 - name: Install nginx
   apt: pkg=nginx update_cache=yes state=present
   tags: nginx, packages