1. A computer system for implementing an application topology on a host computer as virtual machines for testing the application topology before deploying the application topology to real computers, comprising: a component that discovers the application topology of computers; a topology store that stores the discovered application topology that includes, for each computer represented by the application topology, an indication of a network to which the computer is connected, an indication of applications that are associated with the computer, and configuration parameters for the applications; a create topology component that executes on the host computer and, for each computer of the application topology, creates, configures, and launches a virtual machine in accordance with the application topology, the launched virtual machine executing on the host computer; an initialize virtual machine component that executes on each launched virtual machine for a computer and initializes applications associated with the computer in accordance with the application topology; a test component for running tests on the virtual machines, the test component executing on each launched virtual machine to test the application topology before deploying the application topology to the computers; and a component that, after the test component runs tests on the virtual machines, deploys the application topology to the computers.