Fix itermittent error installing Juju

Fix itermittent error installing Juju

We originally made the decision to disable apt update and upgrades for
faster machine provisioning, but we've experienced the scenario where
the provisioning of a container will fail because the apt cache in the
lxd image is stale. This can cause the cloud-init used by Juju to get
stuck in a loop trying to install packages that no longer exist in the
Ubuntu Archive.

This patch enables the run of `apt-get update` when Juju provisions a
container. This will add some time to the provisioning process, but
ensures greater reliability in the process.

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