Add test that generated `cfg.toml` for Homu is a valid TOML file

[aneeshusa]

Buildbot has a checkconfig command to check that the master configuration file is valid, which we test in tests/sls/buildbot/master/config.py. AFAIK there is no similar command for Homu yet, but it should be possible to check that the cfg.toml file parses as TOML at the least using a Python TOML library. This should help prevent issues like the one addressed in #405.

A more comprehensive alternative that requires more work is creating such a checkconfig command for Homu, and writing a test that uses that instead.

Files:

• Create the tests/sls/homu/ folder and make a new Python file inside with the test. Also include an empty __init__.py file in that folder.
• Add a pinned version of the toml library to the requirements.txt file, so it is available on Travis. This is the library used by Homu; ideally, this is pinned to the same version that Homu uses.
• Update the .travis/dispatch.sh file to run the new test on the servo-master1 node.

[Crimack]

Hey, I've made an attempt at doing this on a fork by using a Python library. Two problems though:

1. I can't actually get the Vagrant file to run so I can test? Maybe I'm doing something silly. I keep getting 'extract_id': undefined method 'split' for ["SALT_NODE_ID=servo-linux-cross1", "SALT_FROM_SCRATCH=true"]:Array (NoMethodError) when I try to run vagrant up
2. Is the homu/files/cfg.toml in the master branch a valid TOML file? I think I've written the test correctly, but it keeps failing.

This is my first pull request to a public repo, so thank you very much for the detailed description of what you wanted done! Made making a first contribution pretty easy :)

[aneeshusa]

Welcome to Servo!

1. The Vagrantfile is actually broken (again!), this is my fault 😅. I'm working on a PR to add a test for it so I don't break it again, but in the meantime the changes in this commit should get you up and running. (You don't need the is_salt_master function.)
2. The homu/files/cfg.toml file in the repo is not valid TOML; we are running it through the Jinja template engine to reduce some boilerplate. However, the end result which is deployed by Salt needs to be a valid TOML file, so you should check that file in the test.

Please go ahead and open a PR with your changes, even if incomplete, to make it easier to provide review comments; we can continue the discussion there.

[aneeshusa]

Fixed by #420.