Add Helix Cloud Properties

[zhangmeng916]

When the participant of Helix is running inside a cloud environment and would like to auto register to its cluster, there are a couple of properties needed, e.g. which function to use to fetch/parse the cloud instance information, what is the source for retrieving cloud instance information, as well as some user defined properties, e.g. http timeout, max retries, etc. We should have a globally available Helix Cloud properties defined so that different classes may use it.