Allow dynamic broker configs to be configured in ZooKeeper before starting broker

At the moment, dynamic broker configs like SSL keystore and password can be configured using ConfigCommand only after a broker is started (using the new AdminClient). To start a broker, these configs have to be defined in server.properties. We want to restrict updates using ZooKeeper once broker starts up, but we should allow updates using ZK prior to starting brokers. This is particularly useful for password configs which are stored encrypted in ZK, making it difficult to set manually before starting brokers.

ConfigCommand is being updated to talk to AdminClient under KIP-248, but we will need to maintain the tool using ZK to enable credentials to be created in ZK before starting brokers. So the functionality to set broker configs can sit alongside that.