Use PSU ID instead of SC ID for PSC site identifier. #3253.

Cases uses PSU ID instead of SC ID for PSC site identifier to accommodate multiple Cases instances against a single PSC instance (since PSC supports multiple sites).

With this change, when the new version is being deployed over existing instances,it will require a simultaneous change to the site configuration in the associated PSC instance. Associated PSC instance should be updated to the PSU ID as Site Assigned Identifier. Currently Site Assigned Identifier is set as SC ID in PSC.