Check if management interfaces set in config

[severindellsperger]

Is your feature request related to a problem? Please describe.
Yes, in the future (as soon as the ConfigController (wemulate/wemulate#49) is implemented).
If no management interface is set in the configuration file, it is not possible to execute wemulate commands.

Describe the solution you'd like
There has to be a route included, where can be checked if a management interface is set.
If a management interface is not set, the API should return all available interfaces.
In addition, the route should allow a POST to set at least one interface as a management interface.

Additional context
I propose we use /api/v1/config/ as route.