Remove --silient-upload option, which is invalid

Python-glanceclient removed the --silent-upload some time ago (I
believe when the client was split out from Glance itself).  It's no
longer valid and needs to be removed.  Otherwise users will see
an error of the form:

glance: error: unrecognized arguments: --silent-upload

Change-Id: I417688872bd8e7c4bac7be13d0b0eb13a030fe29