cloudconnector: OAuthClient usage for Dropbox

* Uses OAuthClient module for DropBox filesystem factory.

Signed-off-by: Eirini Psallida <eirini.psallida@cern.ch>
Co-authored-by: Jiri Kuncar <jiri.kuncar@cern.ch>
installation: Pillow instead of PIL

Signed-off-by: Adrian-Tudor Panescu <adrian.tudor.panescu@cern.ch>