Use dedicated env var to enable Incubator features

**Describe the desired outcome from the user's perspective**
As devops, I want a clearer indication of when a zilla deploy is using incubator features or not.

**Acceptance criteria**
 - Recognize env variable `ZILLA_INCUBATOR_ENABLED`
 - set the `-Dzilla.incubator.enabled=true` java opt for the zilla run script based on the env var value
