Make base_url a mandatory config parameter

[leoschwarz]

While this is a breaking change it is relatively small, since it can be quickly addressed even before the update.
It simply will make it required to put the base_url key for each configuration environment in the ~/.bfabricpy.yml file as follows:

    base_url: https://fgcz-bfabric.uzh.ch/bfabric

under your ~/.bfabricpy.yml file's configuration sections (e.g. PRODUCTION).

[leoschwarz]

In general it might also be preferable to define the environments under a "environments:" key instead of doing it at the root level, but it would require a migration path. Leaving this here since it is a related issue.