Data loading: read parameters from a config file

[extua]

In an attempt to make json_adder portable, move all the parameters into a config file, which is read at runtime.

[extua]

Possibly use environment variables when this is passed through to a container.
In the meantime, this can be added to config.yaml when using python as a data loader

https://github.com/freifunk/vis.api.freifunk.net/blob/a3e576e336629bbccb9f3fadf44f510120f0f665/json_adder/python/config.yaml