Enable verbose logging through generate-manifest.sh (antrea-io#1142)

This patch enables verbose logging for antrea-agent and
antrea-controller when generate manifest. This will help in
troubleshooting with increased log level.

hack/generate-manifest.sh

@@ -167,7 +167,7 @@ if [ "$MODE" == "release" ] && [ -z "$IMG_TAG" ]; then
     exit 1
 fi
 
-if [ "$MODE" == "release" ] && $VERBOSE_LOG; then
+if [ "$MODE" == "release" ] && [ ! -z "$VERBOSE_LOG" ]; then
     echoerr "--verbose-log works only with 'dev' mode"
     print_help
     exit 1