It is totally possible for a neutron network to have a network with a
globally routable IPv6 subnet and an RFC-1918 IPv4 subnet. In fact, the
existing OSIC Cloud1 does this, and the original region of Dreamhost did
this.
The trouble is, it's not possible in a reasonable way to _infer_ this
setup, so we rely on brand-new config functions in os-client-config to
allow a user to express that a network is external for ipv4 or for ipv6.
Depends-On: I40f5165d36060643943bcb91df14e5e34cd5e3fa
Change-Id: I12c491ac31b950dde4c1ac55860043fd9d05ece8