Host Affinity - Fix sporadic failures in Container Allocator tests

Related to SAMZA-893, some of the Container Allocator unit tests have been giving us trouble.

For instance; although the HostAwareContainerAllocator expiration logic was broken, the corresponding test (TestHostAwareContainerAllocator.testExpiredRequestHandling()) did not fail.

We've also had some of the other tests fail sporadically in continuous integration testing.

This ticket is to fix the above test in particular, but the ContainerAllocator tests in general. They don't seem to deal with the various threads properly.