Refactoring Driver & Provisioner Base specs to be more similar.
Add command specs for Kitchen::Provisioner::Base.
Extract Kitchen::Configurable mixin to handle common config behavior.

This mixin encapsulates class behavior such as:

* default_config
* expand_path_for
* required_config

The behavior was extracted and generalized from Kitchen::Driver::Base
and Kitchen::Provisioner::Base.
Refactor Provisioner & Driver bases to use Configurable mixin.