override lambda env vars

[andreineculau]

https://github.com/tobiipro/aws-lambda-proxy/blob/master/src/handlers/local.js#L22 and https://github.com/tobiipro/aws-util-firecloud/blob/master/src/lambda/env-ctx.js#L127 makes it impossible to override lambda's env vars e.g. LOG_LEVEL

for the time being, I reckon that it is by design (lambda's env vars should not be overridden; otherwise the logic becomes even more complicated and error prone).
if it's not by design, we might want to allow this

cc @tobiiasl @IanSavchenko