Fix types in test_ec2_ids_not_found_are_printable

This test fails on PostgreSQL when trying to compare
the VolumeIdMapping.id column (Integer) with a passed
string value.

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

Change-Id: Ie529d25029667a5279f9c70f0f3f081b87fb6acb
Merge "Fix types in test_ec2_ids_not_found_are_printable"