Network group unit test

In addition to the integration tests in the tests/cucumber directory, we need to have unit tests that can quickly test specific parts of Wakame without having to wait for the slower operations like starting instances. I made a features directory in dcmgr so we can "require" certain parts of Wakame and test them separately.

I also created a first unit test for the NetworkGroup scheduler. In addition to unit testing, the feature file is also great documentation about how the scheduler works and what configuration it requires.

Use the following command in wakame-vdc/dcmgr to run the unit test.

  bundle exec cucumber