Update documentation to clarify Image Upload setting

The documentation at http://docs.openstack.org/developer/horizon/topics
/deployment.html#file-uploads states that the end user should change the
setting "HORIZON_IMAGES_ALLOW_UPLOAD = False" to disable image uploads.

This suggests that image uploads will be completely disabled, however it
will still be possible to upload images via URL.

This change adds this clarification to the documentation.

Closes-Bug: 1215921
Change-Id: Ie1cc51aaf459f9eb2881121b405fb16e762e265b
Merge "Update documentation to clarify Image Upload setting"