AGPUSH-1107 : Refactored Installations retrieval to work with roles

To test :
* Create App and Variant with developer role
* Add some installations
* log out
* log in as admin
* Verify you can see the developer's installations.

@abstractj @matzew @edewit Could you review/merge ? 
Since the body of `findInstallationsByVariant()` and `findInstallationsByVariantForDeveloper()` is very similar (besides the predicates), can not one of them call into the other one?