A Scheduler Filter to Limit Number of Projects or Tenants in a Host

We need to provide a way to limit the number of projects or tenants that have created instances on a host. Hence this filter will provide support to avoid hosts that have reached the configurable maximum number of projects/tenants.

This blueprint addresses a customer use case where certain hardware systems may have finite
resources that are consumed on a per tenant basis. Allocating tenants above
this limit will fail due to resource exhaustion. These limits may be at the
physical host level or they may be at the network level backing the host
(ie. the TOR). In this scenario, this filter aims for addressing at the
physical host level by enforcing a limit on the number of tenants, while there
is another blueprint which adds the per host aggregate aspect.