Support Stateful and Stateless DHCPv6 by dnsmasq

* This patch adds support for subnets created with 'ipv6_address_mode'
  set to 'dhcpv6-stateful' or 'dhcpv6-stateless' by dnsmasq.
* If no dnsmasq process for subnet's network is launched, Neutron
  will launch new dnsmasq process on subnet's dhcp port in 'qdhcp-'
  namespace. If previous dnsmasq process is already launched,
  restart dnsmasq with new configuration.
* Neutron will update dnsmasq process and restart it when subnet
  gets updated.
* This patch enforces the version check of dnsmasq. dhcp-agent will
  fail to start if version of dnsmasq<2.63.

DocImpact
UpgradeImpact
Blueprint dnsmasq-ipv6-dhcpv6-stateful
Blueprint dnsmasq-ipv6-dhcpv6-stateless

Change-Id: I30e9950bbc5a89f01ccb9c561471f155a9fd1d11
Merge "Support Stateful and Stateless DHCPv6 by dnsmasq"