* adds a service_plugin called segments to enable routed provider
networks
* adds a nova placement api section and inclusion of that section to
neutron.conf
* routed provider networks can be used for setups with and without
charm-neutron-gateway - in both cases there should be a dhcp agent per
segment which can be achieved via charm-neutron-openvswitch
configuration option enable-local-dhcp-and-metadata in case of a setup
without charm-neutron-gateway
Change-Id: I78222b567c72c03ab2d861836172032d4d9a0b3f
Closes-Bug: 1743743
f131505576