support default bundles

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