DT: allow installing unsigned installers with registry key set.

in Deployment Toolkit plugins, only a signed installer can be run after signature verification.
We would like to add a registry key to allow unsigned installers to be run after the error is displayed. The builds are not signed untill promotion, and we need to test the ability to specify a kernel installer, and interaction with the kernel using the setAdditionalPackages() method. This testing could be done before promotion if we add this registry key.