Allow .uproject files to force their project to treat their associated engine as an installed build.

We have a number of small UE4 projects running at any one time and can’t use UGS. Because of this we have to really rely on an installed build made from our dev build that has “projects” that are associated with. This works. but it can be a real pain/time sync with the generation and P4 updates. It would be really nice to be able to treat our engine drop as a P4 distributed build but have each stand alone project look at it as an install build.