engine-iso-uploader - small usage's output change

The usage printed to the screen now says that upload option requires
FILE as mandatory parameter.

Change-Id: Ibf303445c4035589322d2b69dbed8d00d11a64f8
Bug-Url: https://bugzilla.redhat.com/785670
Signed-off-by: Sandro Bonazzola <sbonazzo@redhat.com>