1. A system for customizing extensible applications, comprising: a computer, including a computer readable medium and processor; one or more servers executing on the computer; an extensible application deployed to the one or more servers, wherein the application includes a plurality of real properties; a plurality of tenants, each utilizing the application at the one or more servers; wherein the extensible application is configured to be customized for each tenant, wherein customizing includes for each tenant, defining one or more virtual properties for the application in a metadata file, and mapping the one or more virtual properties to the application using the metadata file.