Revamp database infrastructure.

Revamp database infrastructure.

* We now use DSN URLs using the sqlalchemy.engine.url.URL
  class to pass these values around.  Parameters are now
  database agnostic; applications are tied to specific databases.
* Remove need for WIZARD_MYSQL_DB in test scripts
* Implement `wizard database` and `wizard remove`
* Prevent database exhaustion
* Added dsn and dsn_file property to deploy.Deployment
* Make the remove/backup/restore scripts agnostic
* Added database property to app.Application
* Added callback support to ArgSchema
* Golfed the Scripts specific code into wizard.sql

Signed-off-by: Edward Z. Yang <ezyang@mit.edu>