Improve the configuration mechanism

[PeterJCLaw]

Currently configuration for the various instances is derived from the pi_macs file -- that's a custom format which gets read by a python script to create a hiera config used by puppet. This isn't great and is very non-obvious. The hiera config -> puppet part isn't bad on its own, but as a whole it's not great.

Alternatives include:

• some better format, still in this repo, still generating the hiera config
• move the config externally so that this repo isn't coupled to events

The latter is complicated by the current coupling of the url being shown to the hostname on the machine (which has operational benefits), though we can probably come up with an alternative if we want to go this route. Somehow hosting this information in the compstate might be a reasonable approach, on the grounds that the compstate is already event-specific, however this information doesn't feel very compstate-y.

There might be other approaches here too -- more thought is needed.