Remove internal store references from migration 015

Migration 015 was accessing a object internal to the swift store driver
and changing its behavior in order to do the migration.  This worked but
it made the assumption that the driver logic would not change without
also changing the migration code.  These two should not be dependent
upon each other.  This patch removes all dependencies on swift from the
migration code.

Fixes bug: 1025746
Change-Id: Ic9559ae809b8784d0992cb5275d3911a725110c8
Merge "Remove internal store references from migration 015"