Description
A not-complete config load-minigraph will leave config DB in a wrong state and later operations might not behave correctly.

Steps to reproduce the issue:

1. Modify /etc/sonic/minigraph.xml so that it contains a syntax error.
2. Run config load-minigraph -y, it will fail because of the syntax error.
3. Fix the minigraph and run config load-minigraph -y again. The command will now hang forever.