fixes COUCHDB-2362 admin and cookie auth in dev cluster

 (Edit: added cookie secret to Jira issue and to patch)
   
Ensure that the cookie secret is the same on all nodes.
    
When running the dev cluster with dev/run and the --admin option,
ensure that the hashed admin password is the same on all nodes.
    
Includes a copy of Armin Ronacher's pbkdf2.py:
- https://github.com/mitsuhiko/python-pbkdf2/blob/master/pbkdf2.py