Add a project.volumes_on_host weigher property for custom goodness function

We need a new property to allow access to a project’s quantity of volumes for each host.
If a user requests multiple volumes, they will potentially be placed by the scheduler
on the same backend with the smallest weight (as they are thin-provisionned).
If the user starts to fill all their volumes at the same time,
it can load heavily the backend and cause a drop in performance for all users, as well as exceeding the fill rate.
We can use this new property to reduce the weight of a backend according to the number of volumes owned by the tenant residing on this backend