sonic-db-cli PING doesn't support to only ping dbs in certain redis server instance.

[BrynXu]

Sonic counts on DBInterface code here to set notify-keyspace-events as AKE for each db. This is done through calling sonic-db-cli in postStart() from database.sh.
It worked well as long as we start redis server from docker-database container for all dbs. Now we start to support sonic voq chassis and have a separate container, docker-database-chassis, to start all chassis dbs in new redis server instance. The chassis container starts before database container, as a result, sonic-db-cli PING wont work for it since not all dbs are available at that time.
The solution is to enhance sonic-db-cli to ping dbs from a given redis server instance, something like this: sonic-db-cl PING -i redis-chassis

[BrynXu]

Further thinking about notify-keyspace-events setting, I think we need a new database attribute in database-config.json, so that we can set different value for different dbs, also instead of implicitly setting in PING, it is better to have SET_KEYSPACE_EVENT command to do that.