define required variables.

 refs #1145 @rgasch @drak please review

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | na
| Fixed tickets | maybe #1145
| Refs tickets  | #1145
| License       | MIT
| Doc PR        | na