Subnet allocation from a subnet pool

Subnet allocation from a subnet pool

Contains API changes, model changes, and logic required to enable a subnet to
be allocated from a subnet pool. Users can request a subnet allocation by
supplying subnetpool_id and optionally prefixlen or cidr. If cidr is
specified, an attempt is made to allocate the given CIDR from the pool. If
prefixlen is specified, an attempt is made to allocate any CIDR with the
given prefix length from the pool. If neither is specified, a CIDR is chosen
from the pool using the default prefix length for the pool.

ApiImpact
Partially-Implements: blueprint subnet-allocation
Change-Id: I59a221f4f434718fb77bd132dbbe1ff50fce4b0c