Devstack plugin image build option quoting fix

Options that are unset should not use '' as their default because it
actually puts them literally into the command which will cause it to
skip parsing args after any empty quotes.

Change-Id: I54c0170bad28adb26df87024bd16f31827702f1e

devstack/plugin.sh

@@ -78,7 +78,7 @@ function build_octavia_worker_image {
     export PARAM_OCTAVIA_AMP_IMAGE_SIZE='-s '$OCTAVIA_AMP_IMAGE_SIZE
     fi
     if ! [ -f $OCTAVIA_AMP_IMAGE_FILE ]; then
-        $OCTAVIA_DIR/diskimage-create/diskimage-create.sh $octavia_dib_tracing_arg -o $OCTAVIA_AMP_IMAGE_FILE ${PARAM_OCTAVIA_AMP_BASE_OS:-''} ${PARAM_OCTAVIA_AMP_DISTRIBUTION_RELEASE_ID:-''} ${PARAM_OCTAVIA_AMP_IMAGE_SIZE:-''}
+        $OCTAVIA_DIR/diskimage-create/diskimage-create.sh $octavia_dib_tracing_arg -o $OCTAVIA_AMP_IMAGE_FILE ${PARAM_OCTAVIA_AMP_BASE_OS:-} ${PARAM_OCTAVIA_AMP_DISTRIBUTION_RELEASE_ID:-} ${PARAM_OCTAVIA_AMP_IMAGE_SIZE:-}
     fi
     upload_image file://${OCTAVIA_AMP_IMAGE_FILE} $TOKEN