Installer should allow external thread pool/factory

Since some tests depend on having some modules installed on the target, I've put together a simple programmatic install step to be run during tests (to ensure we have the artifacts we need).

the problem is that I need to know when the install process finishes, and ideally, have an optional blocking operation for a book install process.

This can be partially solved by putting some method on the installer which join() all the worker threads, but ideally, I should be able to specify an external threadpool/worker.