Fix upgrades in cached lxd image creation

A recent update to vmware-tools in trusty caused dpkg to prompt the user
to keep their existing configuration or use the new version, which
caused the update-lxd-image script to fail.

This instructs dpkg to choose which configuration file to keep,
defaulting to the existing configuration.

Signed-off-by: Adam Israel <adam.israel@canonical.com>

jenkins/VCA/update-lxd-image.sh

@@ -39,7 +39,7 @@ function cache() {
     lxc exec $container -- bash -c "while [ ! -f /var/lib/cloud/instance/boot-finished ]; do sleep 1; done"
 
     lxc exec $container -- apt-get update -y
-    lxc exec $container -- apt-get upgrade -y
+    lxc exec $container -- apt-get upgrade -y -o Dpkg::Options::='--force-confold'
     lxc exec $container -- apt-get install -y $PACKAGES $2
     lxc exec $container -- pip3 install --upgrade pip
     lxc exec $container -- pip3 install --upgrade $PYPI