Make example data configurable

[wagner-intevation]

The customer wants to use some example data (data.py) that is not covered by RFC 1918 / RFC2606 / Reserved ASNs, but better known internally.

We can add a configuration parameter that allows to overwrite some example data fields.

[wagner-intevation]

Currently, serve.py imports EXAMPLE_CERTBUND_EVENT from data.py and uses it at some places.

• Add a new optional configuration parameter example_data_file, which accepts a path to a JSON file and document it
  • The data format is a dictionary with one allowed field: example_certbund_event
• At application startup, check if the parameter is set and non-empty
  • If the given file does not exist or is not readable, raise an exception (because that's an erroneous configuration)
  • Otherwise, load the data from the file and create an Event from it.
  • The final example event is EXAMPLE_CERTBUND_EVENT | example_data_file['example_certbund_event'], i.e. EXAMPLE_CERTBUND_EVENT serves the default values and the example_data_file may only provide partial event data