obfs4 bridges should autogenerate keys when invoked with no arguments.
After talking with certain people at the dev meeting, I am convinced that having obfs4proxy autogenerate keys when invoked with no arguments when running as a bridge will be a good idea. I anticipate that this will be less problematic than the similar codepath in scramblesuit as the code has no need to support unmanaged mode.
The current plan is to store the keypair in the pt state directory in what will probably be JSON format so that it is easy-ish for bridge operators to obtain the bridge line for clients at a later date