Consider restricting more the env var that are being set for script execution

[andywaltlova]

#66 introduced the fix to set all RHC_WORKER_* environment variables in the script execution. Currently the variables can get to the script only through it's yaml envelope (they are passed to the script by rhc-worker-script). The variables are in internal repository and changes are subjected to review.

It's not ideal, if someone runs the script without worker it is easy to inject some malicious variable to the runtime.

[andywaltlova]

Now env vars can also be taken from worker config, so two ways now:

• yaml envelope containing the script
• rhc-worker-script config, and any variables passed by rhcd to worker

But we still validate the prefix