Enable CastAsCall for test_api_samples

Like the EC2 tests, the API sample tests are pretty much full-on
integration tests.

However, unlike the EC2 tests, the API tests currently don't use
CastAsCall. Without CastAsCall enabled, it seems that a bunch of issues
are masked simply because the test exits before some RPC casts ever get
executed.

The previous commits fix the cases where CastAsCall causes a failure.
This commit just enables it. Hopefully, it will ensure issues don't get
masked in future.

blueprint: oslo-messaging
Change-Id: Ib01797cec76c7cb3298808636e1b27fec4175594
Merge "Enable CastAsCall for test_api_samples"