Add relative path support for the GIT fetcher

[bdwyertech]

Adds support with TestKitchen for InSpec's recently released relative path feature.
inspec/inspec#4217

Lets you do something like this:

suites:
  - name: default
    run_list:
      - recipe[test::default]
    verifier:
      inspec_tests:
        - test/integration/default
        - name: my_other_cookbook
          git: git@git.mycompany.net:TEAM/chef-cookbooks/my_other_cookbook.git
          tag: master
          relative_path: test/integration/default

[danielcbright]

I've tested this locally, this is awesome! great functionality!
Great job @bdwyertech

[clintoncwolfe]

Thanks!