Add unit testing

[trawler]

We should add unit testing to test the proper propagation of variables from the installer to terraform, and avoid using static terraform variables in testing: #34

go test should:

• accept a yaml data structure in the same format of cna-installer.yaml
• succeed in running terraform cluster create --noop (when expected)
• fail to run terraform cluster create --noop (when expected)

Would allow us to work with https://github.com/actions/setup-go