Update apt after proxy config is dropped

Added an apt update after the apt proxy configuration file is
dropped. The error that was seen is that certain packages could
not be authenticated. Updating apt after the proxy is configure
solves the issue.

Change-Id: Ib971231f81442b74868e692d7d47b1ce827e9c7d
Closes-Bug: #1649416

playbooks/common-tasks/package-cache-proxy.yml

@@ -24,11 +24,18 @@
   copy:
     content: 'Acquire::http { Proxy "{{ repo_pkg_cache_url }}"; };'
     dest: "/etc/apt/apt.conf.d/00apt-cacher-proxy"
+  register: apt_proxy_dropped
   when:
     - proxy_check.status == 200
     - ansible_os_family == 'Debian'
     - repo_pkg_cache_enabled | bool
 
+- name: Update apt when proxy is added
+  apt:
+    update_cache: yes
+  when:
+    - apt_proxy_dropped | changed
+
 - name: Remove apt package manager proxy
   file:
     dest: "/etc/apt/apt.conf.d/00apt-cacher-proxy"