Pretty print json output when writing to a file

It's the opposite of fun to read a single line of json output
when generating the graph results, so if using the -o option
to write to a file, indent the output for readability.

Change-Id: I091ee2ade51def2d59cdeadb792e56746f8882a2

elastic_recheck/cmd/graph.py

@@ -231,7 +231,9 @@ def main():
         out = sys.stdout
 
     try:
-        out.write(json.dumps(jsondata))
+        # indent the json output if we're writing to a file
+        indent = 4 if args.output else None
+        out.write(json.dumps(jsondata, indent=indent))
     finally:
         out.close()