Allow project_id=None for enforce/calculate

This allows a caller to pass None for the project_id if it only wants
it to check the registered limit for a given resource. This is useful
for non-project-scoped resourced where we just want to make sure some
global limit hasn't been exceeded. This would also be relevant for
resources that are created by system-scoped users, such as host
aggregates.

Change-Id: I5fea0143b6a96b5f79bc273961e3e284a260e25e