Add support for passing keyword arguments to BuildFactory.addStep.

This was supported in old versions of buildbot, but there wasn't a release that
supported both, making transitioning harder than necessary.
Revert 'try_client_available_builder_names'.

This added ad-hoc support for overriding `getAvaliableBuilderNames` in a
subclass of `TrySchedulerer`. But that support was undocumented and untested.

This reverts commit 46a81edbecee41e9042b7d1bbf928e13d59e618d, reversing
changes made to 9ec2372129b48842b5c3bc5e933b28ed728d46e3.
Use `assertTrue`/`assertFalse` instead of `assertIdentical` where appropriate.
Merge 'support-depercated-addStep': Add support for passing keyword arguments to BuildFactory.addStep.

We deprecated this in 0.8.6 and dropped support in 0.8.7. But 0.8.7 was the
first version to support not needing to use .addFactoryArguments. But, passing
keyword arguments to .addStep also avoid this.
Don't use deprecated name SetProperty in regression tests.