Add support for default address pool configuration

# Default Subnet Pool to be used for IPv4 subnet-allocation.
# Specifies by UUID the pool to be used in case of subnet-create being called
# without a subnet-pool ID.  The default of None means that no pool will be
# used unless passed explicitly to subnet create.  If no pool is used, then a
# CIDR must be passed to create a subnet and that subnet will not be allocated
# from any pool; it will be considered part of the tenant's private address
# space.
# default_ipv4_subnet_pool =
 
# Default Subnet Pool to be used for IPv6 subnet-allocation.
# Specifies by UUID the pool to be used in case of subnet-create being
# called without a subnet-pool ID.  Set to "prefix_delegation"
# to enable IPv6 Prefix Delegation in a PD-capable environment.
# See the description for default_ipv4_subnet_pool for more information.
# default_ipv6_subnet_pool =