Update Couch DB users and databases

Currently we have three couchdb users:

• admin
• webapp
• ca_daemon

We're not using ca_daemon anymore since we have two different CAs now and the one for client certs just runs on the webapp server.

Instead we need a new user that can only access the sessions table for now. This is used for authenticating other services like soledad. So my proposal is calling that user auth

(from redmine: created on 2013-03-14, closed on 2013-04-24, relates #2178 (closed))