Convert storage drivers to return models

Update the storage drivers to return instances of models classes
instead of dictionaries to ensure that they always return the same
values.

The get_volume_sum(), get_volume_max(), and get_event_interval()
methods are not modified because there will be another changeset
later replacing the use of those methods with calls to
get_meter_statistics().

blueprint storage-api-models

Change-Id: I79da8cc69f70a4f4c24aa13e6a5d2982011eee88
Signed-off-by: Doug Hellmann <doug.hellmann@dreamhost.com>
Merge "Convert storage drivers to return models"