Add support to run balancer overriding current config

At times we want a one-time run for balancer with specific aggressive configs such as recovering from a failure of half cluster. Currently some config are loaded dynamically but some don't. And it could be error prone when we try to restore the configs.

With a dry run feature coming, we want to let user run a dry run with the overriding config to choose the config and then run it. The implementation becomes more straightforward if we allow overidding configs for both config and actual runs.