Load directory from single inspec directory

[chris-rock]

As highlighted in chef/chef#4606 we need to support single inspec subdirectories as well.

acceptance git:(master) tree
.
├── Gemfile
├── README.md
├── basics
│   └── test
│       └── integration
│           └── chef-current-install
│               └── serverspec
│                   ├── chef_client_spec.rb
│                   └── spec_helper.rb
├── top-cookbooks
├── trivial
│   └── test
│       └── integration
│           └── chef-current-install
│               └── inspec
│                   └── chef_client_spec.rb
└── windows-service
    └── test
        └── integration
            └── chef-windows-service
                └── inspec
                    └── chef_windows_service_spec.rb

16 directories, 6 files

To fix that we need to add:

• add a new test suite to our integration test, that have a single inspec directory
• improve legacy mode detection in https://github.com/chef/kitchen-inspec/blob/master/lib/kitchen/verifier/inspec.rb#L68 to catch this case

[alexpop]

Fixed by PR #55