SqlRS: Add read only parameters for information collected by Get-TargetResource

Details of the scenario you tried and the problem that is occurring:
PR #220 added some read-only properties that could be added to xSQLServerRSConfig if they give value.

    [Read, Description("The Windows service name.")] String ServiceName;
    [Read, Description("The SQL database name")] String DatabaseName;
    [Read, Description("Virtual directory name for the report manager.")] String VirtualDirectoryReportManager;
    [Read, Description("Virtual directory name for the report server.")] String VirtualDirectoryReportServer;

The DSC configuration that is using the resource (as detailed as possible):
n/a

Version of the Operating System, SQL Server and PowerShell the DSC Target Node is running:
n/a

What module (SqlServer or SQLPS) and which version of the module the DSC Target Node is running:
n/a

Version of the DSC module you're using, or 'dev' if you're using current dev branch:
Dev

Locate the xSQLServerRSConfig resource and its Get-TargetResource entry point, then review how the existing read-only properties are declared and populated. Add the four listed properties if they provide value, and verify that Get-TargetResource returns them consistently.