Make host, port, and tag can be overridden.

Priority:
env.DROONGA_ENGINE_DEFAULT_DATASET => "defaultDataset" option
env.DROONGA_ENGINE_HOST => "hostName" option
env.DROONGA_ENGINE_PORT => "port" option
env.DROONGA_ENGINE_TAG  => "tag" option
env.DROONGA_ENGINE_RECEIVE_HOST => "receiveHost" option
env.DROONGA_ENGINE_RECEIVE_PORT => "receivePort" option