Add project distribution weigher to support best effort affinity/anti-affinity

We've already had affinity / anti-affinity filters out there to enforce VM placement policy.

However, as a deployer, I may want to:
* spread the VM from the same project in best effort manner, versus a hard requirement in anti-affinity filter solution
* configure best effort anti-affinity as a default deployment option, instead of server group to be an user facing feature

This blueprint is proposing add project distribution weigher, to support best effort manner affinity/anti-affinity placement decision.