Support multiple event hub and storage account sinks for diagnostics …

[yantang-msft]

This code change has gone through diagnostics team's internal code review.

 For PaaS, future schema change doesn't require code change.
 For IaaS, since there is no easy way to convert the xml config to json config, so if there is schema change in the future, we still need to update the code. But the effort is minimized to only update the private schema file.