Description of problem:
The gateway_ip attribute for the isolated networks are not accurate. It is always set to the first address in the cidr. 

In the case of external network the gateway should be set to the value specified in the ExternalInterfaceDefaultRoute parameter. 

ExternalInterfaceDefaultRoute: 2001:db8:fd00:1000:dead:beef:cafe:f00
stack@instack:~>>> neutron subnet-show external_subnet | egrep "allocation_pools|cidr|gateway"
| allocation_pools  | {"start": "2001:db8:fd00:1000::02", "end": "2001:db8:fd00:1000:ffff:ffff:ffff:fffe"} |
| cidr              | 2001:db8:fd00:1000::/64                                                              |
| gateway_ip        | 2001:db8:fd00:1000::1                                                                |

In the case of the other isolated networks there should be no gateway_ip set:

stack@instack:~>>> neutron subnet-show storage_subnet | egrep "allocation_pools|cidr|gateway"
| allocation_pools  | {"start": "fd00:fd00:fd00:3000::10", "end": "fd00:fd00:fd00:3000:ffff:ffff:ffff:fffe"} |
| cidr              | fd00:fd00:fd00:3000::/64                                                               |
| gateway_ip        | fd00:fd00:fd00:3000::1                                                                 |

Version-Release number of selected component (if applicable):
openstack-tripleo-heat-templates-0.8.6-112.el7ost.noarch

This doesn't affect functionality since we're assigning the addresses statically on the overcloud nodes.