Add `make lint` target and fix errors that were found

Change-Id: Ibb2886d07e0e1c8a4b8928721a12eb94c01178a0
diff --git a/Makefile b/Makefile
index 2d8c2ba..74377f4 100644
--- a/Makefile
+++ b/Makefile
@@ -14,6 +14,9 @@
 $(JOBCONFIG_DIR):
 	mkdir $@
 
+lint:
+	yamllint -c yamllint.conf jjb/
+
 test: $(VENV_DIR) $(JOBCONFIG_DIR)
 	source $(VENV_DIR)/bin/activate ; \
 	pipdeptree ; \