Allow user to override the Application environment

[veedo]

In order to pass a different environment into the remote node when starting it, the user must change their local Application env, start the node, then change it back. For simple cases this works fine, but once tests are running in parallel, this can cause chaos.

This PR adds an :env_override option that allows the user to inject different environmental variables into the remote node. This is especially useful when each group of nodes in a test must start up on a different port or with different app options enabled.

Build passed: https://travis-ci.org/github/whitfin/local-cluster/builds/667949124

[whitfin]

I'm not sure I understand the use case; why would your test environment have different options to the environment you're running your tests under? What's the use case that you can't just change them in your local env?

[whitfin]

Regardless, I merged your changes inside https://github.com/whitfin/local-cluster/commit/523545b0bdc7cb077e8afb080d01ed35d4e53a8e - with a tweaking of the naming to environment and a slight touch of reformatting. Thanks for the contribution!

[veedo]

Thanks

Some examples that I'm using this for:

• Testing that simulates nodes across multiple devices. Different devices often have different configs to tune behaviour.
• Testing conflicting configs. i.e. What happens when one device gets updated and joins the cluster.