Allow breaking out from the Beiboot cluster into the host cluster

[Schille]

The following use-case:

• Host cluster runs some vital services centralized (with prod parity)
• Beiboot cluster runs apps that consume these centralized services (or only an excerpt of the whole platform)

How to reach them? Probably we need some kind of proxy for the outer services to become available for the Beiboot cluster-based resources.
Examples of these centralized services are:

• a huge database management system that holds data for my app running in Beiboot that I don't want to replicate (or is infeasible or too expensive)
• a service that holds certain resource constraints, such as a static IP or certificates

@tschale what do you think?

[Schille]

Getdeck "virtual-host" bridge.