Add IPv6 support for tenant data network

Define IP_VERSION with one of the three values 4, 6, or 4+6 in
your localrc to indicate if you intend to run your tenant data network
as either IPv4, IPv6, or dual stack respectively. Default value is 4.

If your IP_VERSION is set to 6 or 4+6, then the following variables
should be defined in your localrc:
  - FIXED_RANGE_V6: The IPv6 prefix for your tenant network
  - IPV6_PRIVATE_NETWORK_GATEWAY: The gateway IP with the same prefix
  - IPV6_RA_MODE (with default as slaac)
  - IPV6_ADDRESS_MODE (with default as slaac)

If you're going to use IPV6_RA_MODE/IPV6_ADDRESS_MODE settings other
than the defaults then you should make sure your VM image has dhcpv6
client enabled at bootup, otherwise you'll need to run it manually
after the VM is booted.

It's recommended to run the latest version of dnsmasq 2.68.
If you intend to enable internet access in your VM, make sure
your network node has IPv6 internet access, and the IPv6 prefix for
your tenant network is a GUA and routable.

Implements: blueprint ipv6-support
Change-Id: I848abf18e00e2a869697c5ef6366bc567dde448a
Co-Authored-By: John Davidge <jodavidg@cisco.com>
Merge "Add IPv6 support for tenant data network"