Plugins should call __init__ of db_base_plugin for db.configure

Currently each plugin calls db.configure() within the plugin's __init__
class or defines an initialize() method that's sole job is to call this
method. Instead we should just call the super method of a plugin so that
db.configure() is called for us out of the db_base_plugin class.

Note: the only reason why I'm making this change is that I want to add
something to the __init__() class of the db_base_plugin that's needed for
the nova-event-callback blueprint and adding it in the base class of init
looks to be the best place.

Change-Id: Iec3c912735021ceb90f657108aad3a57460d66e7
Closes-bug: #1282303
Merge "Plugins should call __init__ of db_base_plugin for db.configure"