Edge NAT w/ASR uses unrouteable subnet

[Thomas Bachman]

With GBP workflow, the unrouteable subnet for the external segment is sometimes used as the primary IP on the ASR's external VLAN sub-interface (i.e. the one that connects to the outside world). This needs to be fixed, such that a NAT pool must be configured/added to an external segment before an L3P is associated with an L3 segment (Edge NAT mode only). An IP address from this NAT pool must be used for the primary IP on the external VLAN sub-interface of the ASR. If multiple NAT pools are configured for an external segment, the subnet to use for the primary IP is ambiguous, so the plugin will select an IP from the NAT pool with the lowest UUID.

[OpenStack Infra (hudson-openstack)]

Change abandoned by Thomas Bachman (<email address hidden>) on branch: master
Review: https://review.openstack.org/417027
Reason: This is the wrong approach -- will document process to use for Edge NAT instead.