Fix upgrades in cached lxd image creation

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>