Removed unused imports in vnfd and nsd yang models, warnings in Pyang shown now as errors

Change-Id: Iafcac411b4a7b5798a8e641464a7bf40312d9aa6
Signed-off-by: garciadeblas <gerardo.garciadeblas@telefonica.com>
diff --git a/Makefile b/Makefile
index c72183e..6305363 100644
--- a/Makefile
+++ b/Makefile
@@ -27,7 +27,7 @@
 RW_PB_EXT := build/yang/rw-pb-ext.yang
 Q?=@
 
-PYANG_OPTIONS := -Wnone
+PYANG_OPTIONS := -Werror
 
 all: $(PYTHON_MODELS) pyangbind
 	$(MAKE) package