Add support for `test_spec.json` for Git storage

[netme]

Every feature added to Dockerfile should have a scenario how to test it. Scenario should be stored in test_spec.json file together with Dockerfile and meta.yml.

Here is an example for Python feature:

[{"feature":"python","cmd":"python -V"}]

Steps:

1. Add test_json files to test repository
2. Implement support for fetching test_json together with features.

[netme]

Current status: test specs are generated, bats is installed, but the tests are not running.