Allow environment variable substitution in config yaml

[eacmen]

Allow variables such as sensitive API keys or other auth tokens to be set as environment variables and substituted in config.yaml when parsed at runtime (i.e. with a $ENV{ENVNAME} type syntax). This is standard practice for cloud-based CI solutions such a Travis, Circle CI, etc.

[benhowes]

In #6 we're looking at adding a means to pass in variables on the command line. I expect that will be the preferred way to pass env variables to tests in tavern.

leaving this open for now, until we move forward with #6 and decide if this is needed as well as that.

[michaelboulton]

I realised this was only a couple of lines of code to do so I have implemented it in this branch https://github.com/taverntesting/tavern/tree/feature/env-var-formatting - not sure what the 'magic' variable name should be for environment variables though? I suppose that it just means people can't save a variable called ENV_VARS

[eacmen]

👍 we'll test it out and let you know of any issues.

[AmiSMB]

I have tested this branch out and it works very well for me as I am using tox with tox-docker which launches my docker container and leaves an environment variable in the form of for any port listed with EXPOSE in the Dockerfile.

So now when tox-docker starts the container, pytest runs with the Tavern plugin I can use the ENV_VARS in the yaml file to get the correct port for the url to test my microservice. Good work 👍

[michaelboulton]

With #36 there is going to be a value called tavern.env_vars that you can format environment keys from