Fix FK violation errors in InstanceActionTestCase

A few tests of InstanceActionTestCase inserts rows,
that have no corresponding values in 'instances' table,
and this causes FK violation errors.

This change fixes InstanceActionTestCase when running
tests on PostgreSQL/MySQL or on SQLite with FKs enabled.

Blueprint: db-api-tests-on-all-backends

Change-Id: I9ec39d5f4c21ba5a627b289164480c694bc1539c
Merge "Fix FK violation errors in InstanceActionTestCase"