fix: report this gem's own version to kitchen diagnose

[tas50]

kitchen diagnose is what bug reports are asked to include, and what it said about this plugin was wrong in both halves.

Before

driver:
  Docker:
    class: Kitchen::Driver::Docker
    version:            # nothing
    api_version:        # nothing
transport:
  Docker:
    class: Kitchen::Transport::Docker
    version: 4.1.1      # Test Kitchen's version, not kitchen-docker's
    api_version:

The transport declared plugin_version Kitchen::VERSION — that constant is Test Kitchen's version, so a diagnose from kitchen-docker 3.3.4 running under Test Kitchen 4.1.1 reported the transport as 4.1.1. The driver declared no version at all, and kitchen_transport_api_version was sitting there commented out.

Every plugin shipped with Test Kitchen declares both (kitchen_driver_api_version 2, kitchen_transport_api_version 1).

After

driver:
  Docker:
    class: Kitchen::Driver::Docker
    version: 3.3.4
    api_version: 2
transport:
  Docker:
    class: Kitchen::Transport::Docker
    version: 3.3.4
    api_version: 1

Risk

None to behaviour. Both values are diagnostic only — nothing in Test Kitchen branches on either; api_version is read exactly once, by Configurable.diagnose. This changes what is reported and nothing else.

Confirmation

Output above is real kitchen diagnose --all against Test Kitchen 4.1.1, before and after.

rake style clean; rspec 319 examples, 0 failures (6 new, including one pinning that the reported version is not Kitchen::VERSION).