1006374: add delete cascade to cp_pool_*

adding in delete cascades on the pool_id foreign keys will allow
deletion of pools and have their provided/derived products go away.

In the event we end up with duplicate entries again in cp_pool_products
this change will prevent the constraint violation when deleting
the pool.
Merge branch 'master' into zeus/1006374

Conflicts:
	src/main/resources/db/changelog/changelog-create.xml
	src/main/resources/db/changelog/changelog-update.xml
Merge pull request #393 from candlepin/zeus/1006374

1006374: add delete cascade to cp_pool_*