Allow visors to advertise themselves as running on Public IP

[jdknives]

Feature description

We should make it easier for users to establish transports automatically. The best scenario would be to make them establish stcpr transports to visors on public IPs. We should allow visors on public IPs to register themselves in the service discovery so other visors can establish transports to them upon startup.

Is your feature request related to a problem? Please describe.
Currently, if visors want to open up a route to another visor, they need to establish transports manually.

Describe the solution you'd like
I would suggest we go with a semi-manual way of solving this issue by allowing operators of visors to set a config value that indicates their visor is reachable for stcpr transports. When this is set, the visor registers itself in the service discovery. Otherwise we would have to automate the detection of whether visor is reachable for stcpr. In any case, we should verify in the service disc whether entries are valid.