Simple subnetpool allocation quotas

Simple subnetpool allocation quotas

Enables enforcement of allocation quotas on subnet pools. The quota
is pool-wide, with the value of allocation_quota applied to every
tenant who uses the pool. allocation_quota must be non-negative,
and is an optional attribute. If not supplied, no quotas are
enforced. Quotas are measured in prefix space allocated. For IPv4
subnet pools, the quota is measured in units of /32 ie each tenant
can allocate up to X /32's from the pool. For IPv6 subnet pools, the
quota is measured in units of /64 ie each tenant can allocate up to
X /64's from the pool. For backward-compatibility, allocation quotas
are not applied to the implicit (AKA null) pool. Standard subnet
quotas will continue to be applied to all requests.

ApiImpact
Partially-Implements: blueprint subnet-allocation
Change-Id: I7e4641f47790414c693c7cc9b7a44b1889087801