merge the latest peer-xplod into core

[jameinel]

Description of change

Peer-xplod currently uses python2 for all of its hooks. Xenial doesn't have it installed by default, so we need to install it in the 'install' hook.
This also lets us change the charm into a multi-series charm, and fixes 'categories' vs 'tags' which is something that 'charm proof' complains about.

It also changes it so if you juju set peer-xplod maximum=0 the charm hooks never stop firing, which was the original intent.

QA steps

You should be able to juju deploy ./peer-xplod --series xenial and have it work.

Documentation changes

No, this is used for internal testing.

Bug reference

No