Update apt cache after adding repos or pinning

This patch implements a forced apt cache update after a new apt
repository is added, or any apt pinning is implemented.

This ensures that apt is sure about what packages are available to it,
and which packages/repositories should be preferred above others.

Change-Id: I2f6e9fa33c7765e8ed8aeaf4ea60be696125c186
Closes-Bug: #1421679

rpc_deployment/roles/common/tasks/apt_pinning.yml

@@ -18,3 +18,7 @@
     src: apt_pinned_packages.j2
     dest: /etc/apt/preferences.d/openstack_pinned_packages.pref
   when: apt_pinned_packages is defined
+
+- name: Update apt cache
+  apt:
+    update_cache: yes

rpc_deployment/roles/common/tasks/repos.yml

@@ -39,6 +39,10 @@
     group: "root"
     mode: "0644"
 
+- name: Update apt cache
+  apt:
+    update_cache: yes
+
 - name: Ensure common Packages are Installed
   apt:
     pkg: "{{ item }}"