Repository.initialize does not handle multiple key dirs

[dennisvang]

Description (and how to reproduce)

For example, in the repo_workflow_example, if keys exist, but the metadata have been removed (for whatever reason), then Repository.initialize() will try to create new metadata, but fails to find the private keys.

The reason is twofold:

• Repository.initialize() only looks in self.keys_dir
• Keys.find_private_key() is not recursive (which would fix this specific case)

Proposed solution

• add an extra_key_dirs arg to Repository.initialize()
• make Keys.find_private_key() recursive