Add an environment variable function

[andrioni]

It'd be nice to be able to support environment variables directly: it helps with the testing and reduces the chances of someone committing credentials :)

[alexanderdean]

Neat idea!

[alexanderdean]

This is less important given #35, but still nice

[jbeemster]

The priority here being:

• Load Env vars into variables map
• If present in playbook use playbook value
• If present in -var use that value

Is that right @alexanderdean ?

[alexanderdean]

Can't we just have a systemEnv("foo") function that checks to see if there is an environment variable available called "foo"?

[jbeemster]

We good as well yeah! Does seem a bit saner than loading all of the environment variables into the application..