RBE node - let topic handling be optional

[dceejay]

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)

Proposed changes

Adds a checkbox (on by default) - to handle msgs with msg.topic set as different streams - thais has always been the default - but some users find it confusing - so this allows them to turn it off and test every message as part of the same test.

Checklist

• I have read the contribution guidelines
• For non-bugfix PRs, I have discussed this change on the forum/slack team.
• I have run grunt to verify the unit tests pass
• I have added suitable unit tests to cover the new/changed functionality

[coveralls]

Coverage increased (+0.2%) to 49.146% when pulling 6f38207 on rbe-topic-optional into 77fac03 on master.