Sample conf for dropbox and owncloud file sources

[abretaud]

What did you do?

• Added try/except to catch errors when listing remote files
• Added sample conf for dropbox and owncloud

Why did you make this change?

(Cite Issue number OR provide rationalization of changes if no issue exists)
(If fixing a bug, please add any relevant error or traceback)

Fixes #11145

How to test the changes?

(select the most appropriate option; if the latter, provide steps for testing below)

• I've included appropriate automated tests.
• This is a refactoring of components with existing test coverage.
• Instructions for manual testing are as follows:
  1. [add testing steps and prerequisites here if you didn't write automated tests covering all your changes]

[jmchilton]

Thanks for continuing to push this @abretaud - I really appreciate your help and the other issues you've created also!