Fix missing ca-certificates package

Occassionally during gate test, some containers
seem to be lacking or have an outdated 'ca-certificates'
package.

This patch adds ca-certificates to container cache prep
for the container build process.

Change-Id: Ib3613e4338e4dc7e2f1df75e842aa4213d207746

releasenotes/notes/add-ca-certs-2398cb4856356028.yaml

@@ -0,0 +1,6 @@
+---
+upgrade:
+  - The ``ca-certificates`` package has been included in the LXC
+    container build process in order to prevent issues related to
+    trying to connect to public websites which make use of newer
+    certificates than exist in the base CA certificate store.

vars/redhat-7.yml

@@ -59,6 +59,7 @@ lxc_cache_map:
 lxc_cache_install_command: "yum install -y"
 
 lxc_cache_packages:
+  - ca-certificates
   - openssh-server
   - python2
   - sudo

vars/ubuntu-14.04.yml

@@ -70,6 +70,7 @@ lxc_cache_packages:
   - bridge-utils
   - bsdmainutils
   - build-essential
+  - ca-certificates
   - cgmanager
   - cgmanager-utils
   - cgroup-lite

vars/ubuntu-16.04.yml

@@ -70,6 +70,7 @@ lxc_cache_install_command: "export DEBIAN_FRONTEND=noninteractive && apt-get upd
 
 lxc_cache_packages:
   - apt-transport-https
+  - ca-certificates
   - openssh-server
   - python2.7