Fix syntax error in script

One of the recent patches introduced a syntax error in the marconi
script. This patch fixes the issue & gets marconi running in
devstack again.

Change-Id: I46a039f72174c4898869983de03a1e29cbe435bf
diff --git a/lib/marconi b/lib/marconi
index 143f38a..20bf0e1 100644
--- a/lib/marconi
+++ b/lib/marconi
@@ -154,7 +154,7 @@
 
 # start_marconi() - Start running processes, including screen
 function start_marconi {
-    if [[ ${USE_SCREEN,,} == "false" ]];; then
+    if [[ "$USE_SCREEN" = "False" ]]; then
         screen_it marconi-server "marconi-server --config-file $MARCONI_CONF --daemon"
     else
         screen_it marconi-server "marconi-server --config-file $MARCONI_CONF"