Fix issue with set-feature-flags task

[davewalter]

What is this change about?

The current SYSTEM_DOMAIN check runs before it can be set by setup_bosh_env_vars, which causes it to fail if the value is being read from an environment metadata file.

Please check all that apply for this PR:

• [ ] introduces a new task
• [x] changes an existing task
• [ ] changes the Dockerfile
• [ ] introduces a breaking change (other users will need to make manual changes when this is released)

Did you update the README as appropriate for this change?

• [ ] YES
• [x] N/A

How should this change be described in release notes?

Fix a bug in the set-feature-flag task introduced in the last release where it would fail if used with a toolsmiths environment.

What is the level of urgency for publishing this change?

• [x] Urgent - unblocks current or future work
• [ ] Slightly Less than Urgent

Tag your pair, your PM, and/or team!

[cf-gitbot]

We have created an issue in Pivotal Tracker to manage this:

https://www.pivotaltracker.com/story/show/171032801

The labels on this github issue will be updated when the story is started.