Allow overriding InterfaceDefaultRoute with ips_from_pool template
Allow overriding InterfaceDefaultRoute with ips_from_pool template

Currently InterfaceDefaultRoute doesn't affect when ips_from_pool
template is used, and gateway_ip in network_data is always picked up.
This patch allows operators to override default route by the parameter
even when ips_from_pool template is used.

Note that this patch also fixes incorrect handling about subnet in
ips_from_pool template when ipv6 is enabled.

Closes-Bug: #1881768
Change-Id: I2f0f60052c1acfc971fe3d5195f9499e45068396

openstack/tripleo-heat-templates,openstack/tripleo-heat-templates