Improve SystemInstallation>>install: flexibility

[gcotelli]

SystemInstallation>>install: now requires every module to be created by implementing the toInstallOn: message. This is Ok for self-contained modules, but sometimes we want a module with a bit of configuration and the only way to overcome this limitation now is to create a "module factory".
We can refactor a bit install: so self-contained modules are easy to install, but also allow more exotic module creation strategies.