Fix edge case + additional debugging

An edge case was detected and fixed: we can't guarantee that the cloud
image used will have an updated apt cache, so run `apt-get update`
before attempting to install the iptables-persistent package.

Second, additional debug output was added to trace an error that's only
been reproduced in the rc1 LCM image published to hub.docker.com.
Building the image locally have been unable to reproduce the error.

Change-Id: I333bedab8d754102eb13b9da06ee278b6cf0d6ad
Signed-off-by: Adam Israel <adam.israel@canonical.com>