Add test coverage

[flavorjones]

You're not a bad person for not TDDing, but we should cover:

• [x] required parameters (graceful failure if not provided)
• [x] environment variable expansion
• [x] BUILD_URL
• [x] output json values, particularly host
• [x] but not emitting password

And we should run the tests as part of the Docker image creation.

[jtarchie]

Aren't you?