This is, like the drivers in zuul, designed to be a single instance
per driver that survives for the life of the process. It is used
to further instantiate driver-specific interfaces.
Here we have it return the config object for the driver (replacing
the previous system which loaded it from specific config files).
We also move the reset method from the ProviderConfig to the Driver
object. It's currently only used to clear a global os client config
object, so this better matches its lifecycle.
Change-Id: I1f5a7be9c597be842bfe4dbea8f153d7a96d7b9a
82d8c51250