Advanced task specific environment variables

[bomoko]

With #185 we mounted the standard environment variables into advanced tasks.

There has been some discussion about supporting advanced task specific environment variables to inject details that we might not want available in, say, a cli pod.

This should be fairly simple to implement - the question is, where and how?
Currently we inject the JSON_PAYLOAD with whatever details come through from the invocation of the advanced task - if we piggy-back on that, we could move the entire logic into uselagoon/lagoon. Alternatively, we could extend the advanced task implementation to support a separate set of variables (also passed with the task information).

Thoughts?