[RFE] Provide a set of default URL for Fedora apps

[kwizart]

This RFE is based on the need to specify an alternative set of default URL for python-fedora based apps.

For example, using an alternative URL for an app (for staging/testing purpose, say "koji hub url" ), this URL would need to be overridden to every single python-fedora based apps and each app would grow an option to override the default URL.

Instead, I would suggest to make python-fedora read a config file describing the whole set of default staging/testing URL when the FEDORA_CLIENT_NAMESPACE=staging environment is set.

That way it would be possible to use any python-fedora enabled apps unmodified.