Add container_count to policy_stat table

Start tracking the container count per policy including reporting
it in account HEAD and supporting installations where the DB
existed before the updated schema.

Migration is triggered by the account audtior; if the database is
un-migrated it will continue to report policy_stats without the per
policy container_count keys.

Closes-Bug: #1367514
Change-Id: I07331cea177e19b3df303609a4ac510765a19162
Merge "Add container_count to policy_stat table"