Fix issue with account broker and pre-SPI databases

Set a DEFAULT storage_policy_index for existing container rows during
migration.

Copy existing object_count and bytes_used in policy_stat table during
migration.

Change-Id: I6b3f591bad455dcd072056c28fc5a37c3f618e40
Implements: blueprint storage-policies
Merge "Fix issue with account broker and pre-SPI databases" into feature/ec