Conductors maintan driver list in the DB

To facilitate exposing a list of drivers in the API,
and keep track of which ironic-conductor instance status,
this patch ...
* adds a new 'conductors' table in the DB
* adds a Conductor object class (with intentionally limited
  functionality)
* adds db/api methods for register/unregister/get/touch
* adds periodic task to conductor to maintain its updated_at field
* adds an additional db/api method to retrieve a list of drivers
  which are registered by active conductor instances

Change-Id: I1ebdb92d5c2d6ad1a6d1717dd13ff51be181ccc0
Merge "Conductors maintan driver list in the DB"