Juju/LXD performance issues

[David]

At F2F 6, the issue was raised again about the time it takes for Juju to start a charm. It's been raised before, with work in progress to address it.

In part, it stems from LXD setup (it may not be using ZFS in many cases) and the removal of scripts from the R3 installer that would pre-cache the image used by Juju when starting a machine.

The pre-cached LXD image:

- refreshes nightly, via cron.daily
- supports trusty, xenial, and bionic
- runs `apt-get update` and `apt-get upgrade`
- installs cloud-init dependencies
- installs dependencies of reactive charm framework
- installs agent for currently installed version of Juju

We were also asked to provide some performance metrics, so the TSC+MDL could make a decision about other performance-impacting options, such as disabling `apt-get update && apt-get upgrade` for every new machine vs. the previous approach of caching that nightly as part of building the juju lxd image.