Improve main function

The main function is wrapped with a sys.exit function[1] which has an optional
argument but the current implementation doesn't return a value in case of a
successful execution. This change ensures that main function returns a value
consumed by sys.exit function.

[1] https://docs.python.org/3/library/sys.html#sys.exit

Change-Id: I3b65db9ce57e53b8696e417bcfbd52c67ddb0b89

bifrost/cli.py

@@ -303,9 +303,10 @@ def main():
         if args.debug:
             raise
         else:
-            sys.exit(str(exc))
+            return str(exc)
     except KeyboardInterrupt:
-        sys.exit('Aborting by user request')
+        return 'Aborting by user request'
+    return 0
 
 
 if __name__ == '__main__':