support wildcard hostnames

Here's my verbatim commit message:

> keter bundles configured with a primary hostname of "*" will serve as
> the default target for all requests that aren't associated with an
> explicit host mapping
> 
> this is useful when running keter as a deployment manager for a single
> application on multiple machines with distinct hostnames behind a
> load-balancing reverse proxy that modifies the http host header:
> cluster-wide deployments no longer require the generation of a unique
> keter bundle for each machine
> 
> managing two separate wildcard applications with the same keter instance
> will result in an obvious race

I feel a little gross about the way I hacked this in (because I'm encoding meaning in an ad-hoc way using a string), but whatevs!