factor out db.py and userdb.py to eduid_am, EDUID-155
Merge remote-tracking branch 'origin/eperez-refactor-mongodb'
Use lambdas to find db instances.

Stop using the get_db and get_userdb from eduid_am -
that code should not know or care how and where we store our
instances.
Update eduid_am to 0.3.0.
Fix use before assignment.

Hopefully correct fix - just found using code inspection.
Semantic versioning, now at 0.2.0.