Cleanup unused imports in test module
Show better error message for invalid JSON

Especially for list params, their error messages were not helpful.
Error messages now include the actual value itself, as we have seen
cases where customers specify valid JSON but the shell they're using
will process arguments and strip out characters such as quotes.
By showing the actual JSON contents it will make this more obvious.

Before:

  The value for parameter "--" must be JSON or path to file.

After:

  Error parsing parameter --block-device-mappings: Invalid JSON:
  [{ ...BAD JSON CONTENTS ...}]
Merge branch 'multi-json-arg' into develop

* multi-json-arg:
  Show better error message for invalid JSON
  Cleanup unused imports in test module
Add changelog entry for #639