Ensure 'status' in update_snapshot_status

The function os-update_snapshot_status doesn't check whether the param
"status" is in request body. It throws 500 error. We should catch the
KeyError and return 400 (HTTPBadRequest) instead.

Change-Id: If3775e3c0299cf0edbdda5081a6633b38462011b
Merge "Ensure 'status' in update_snapshot_status"