Use fixed router in the OpenTripPlanner plugin
Currently the OpenTripPlanner plugin tries to obtain a list of "routers" (different regions corresponding to distinct graph objects internally in the server) by querying the /routers REST call. We should change this to use a fixed router ID per plugin instance (would default to use the "default" pseudo ID which is referring to the first router found (or the only one in such cases) for a few reasons:
- OTP will probably remove support for running several graphs in the same instance in version 2.0.
- Doing this now is a redundant as our service discovery will describe regions and the plugin instance to use (since #199 (closed)).
- Some services don't expose the /routers call (and thus will not work with our current implmentation, such as the Finnish service (see https://digitransit.fi/en/developers/apis/1-routing-api/ and https://api.digitransit.fi/routing/v1/routers/finland/).