Add Doctor Checks to Driver and Verifier

[aaron-lane]

This PR adds doctor checks to the driver and verifier, specifically to check that driver.client does exist and is executable, and that verifier.systems is not empty. This PR partially implements #212.

[aaron-lane]

kitchen doctor only calls #doctor on the first plugin that it detects with an implementation: https://github.com/test-kitchen/test-kitchen/blob/v3.5.0/lib/kitchen/instance.rb#L244. The logic needs to be centralized and generalized to be executable by all the plugins.