Updated 'nat' role

'nat' role now supports new 'interfaces' network configuration.
Add default search domain in NAT interface config
Update 'dnsmasq' role

- dnsmasq now has more configuration options exposed via Ansible
  variables (default local domain, subdomain to use in NAT, default MX
  host, list of CNAME domains, and others), which can be configured in
  inventory;

- by default, dnsmasq enables PXE boot on the gateway host, and
  downloads Debian Installer netboot image to allow easy Debian
  installation over PXE. This can be disabled with dnsmasq_netboot
  variable set to False;
Update playbooks/local_nat.yml (mostly cleanup)