Use redis hset instead of set for CONFIG_DB_INITIALIZED

[jipanyang]

Similar to portTable ConfigDone, it will be good for py swsssdk to be consistent with existing c++ swss-common library to use hset instead of set for CONFIG_DB_INITIALIZED.

The flag is to be checked by all app modules which need access to configDB. Using redis operation which is not supported by c++ swss-common library causes unnecessary extra work in c++ modules.