Fix #7057 missing False default for vars_prompt

The test for default should be "is not None" since a boolean value of
False is a valid value.

See discussion at:
  https://groups.google.com/forum/#!topic/ansible-project/oc1_zE-FnyI
Merge pull request #7058 from jkleckner/fix-vars_prompt-when-default-False

Fix #7057 missing False default for vars_prompt