fixes COUCHDB-2362 admin and cookie auth in dev cluster

  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