Filter to limit number of projects or tenants in a host aggregate

We need to provide a way to limit the number of projects or tenants that have created instances in hosts belonging to a host aggregate. Hence this filter will provide support to avoid hosts, whose parent host aggregates have reached their configurable maximum limit. A key called max_projects will be needed to be specified as the metadata for the individual host aggregates that need this limiting.

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 host
aggregate level (i.e., all hosts under a TOR), by enforcing a limit on the
number of tenants, while there is another blueprint which captures on a per
host level.

Note this is different than the aggregate tenant isolation, where specific tenant ids are used, here
this filter is just support a check for just the number of tenants that can create instances in host aggregates.