Add support for using the default subnetpool
Add support for using the default subnetpool

Allows to pass use_default_subnetpool instead of a cidr. This is
required when adding an IPv6 subnet if BGP or prefix delegation is
enabled.

Change-Id: Icf7c212ee730d631599fba732f66679c5cd2916d

dtroyer/python-openstacksdk,dtroyer/python-openstacksdk,openstack/python-openstacksdk,openstack-infra/shade,openstack-infra/shade,stackforge/python-openstacksdk,openstack/python-openstacksdk,stackforge/python-openstacksdk