Changes to load Image Streamer auth settings from fixture

[marikrg]

Description

Enable run Image Streamer unit tests with no auth files/environment variables set by the user.

Issues Resolved

103 (partial, fixes the issue running Image Streamer unit specs)

Check List

• [x] New functionality includes testing.
  • [x] All tests pass ($ rake test).
• [ ] New functionality has been documented in the README if applicable.
  • [ ] New functionality has been thoroughly documented in the examples (please include helpful comments).
• [ ] Changes are documented in the CHANGELOG.