Update nightwatch preview command

[lilyzh]

Status: Ready for Review Owner: Reviewers: @ellenmobify @marlowpayne @mobify-derrick

Changes

• Add the other siteConfig in preview command

  Todos:

• [x] Passed linting check (run grunt lint)

  Feedback:

none so far

How To Test

• Checkout this branch
• npm install
• grunt lint

[marlowpayne]

👍 from me! :shipit:

If these changes need to ship, make sure to follow our branching strategy for releasing a new version. @mobify-derrick @ellenmobify or myself can help with that if needed.