Fix a bug where services that were already installed on a cluster
were getting planned to be installed again when adding a service
that depends on it to the cluster. For example, if service A is
on the cluster and we add service B, whose install requires
service A.
completing the refactoring that was only halfway done.
Merge pull request #189 from continuuity/feature/planner-wrong-when-adding-services

Feature/planner wrong when adding services