Hide _flush() output on the remote_storage page

Redirect stderr and stdout to /dev/null so the screen doesn't get
cluttered.

Change-Id: I7ba2797b43455ac621f780963bf5eaf7a24a1219
Signed-off-by: Ryan Barry <rbarry@redhat.com>
Signed-off-by: Fabian Deutsch <fabiand@redhat.com>