sdn: dhcp: add dnsmasq plugin

sdn: dhcp: add dnsmasq plugin

The plugin creates a dnsmasq@<zone> service that spawns a dnsmasq
instance that handles dhcp for that zone.

The configuration files for a dnsmasq instance lie within
/etc/dnsmasq.d/<zone>

The plugin generates the following configuration files:
* 00-default.conf
  Contains the default global configuration for dnsmasq. Disables DNS,
  enables some specific options for Windows, does some
  security-related configuration and makes dnsmasq bind only to the
  interfaces it is responsible for
* 10-<subnet>.conf
  Contains the default settings for a subnet. Sets dhcp options for
  DNS and gateway.
* 10-<subnet>.ranges.conf
  Contains the configuration of DHCP ranges for a subnet.
* ethers
  Contains the MAC address to IP mappings for a zone.

Every subnet gets assigned a tag in the dnsmasq configuration that is
equal to the subnet_id. This can be used to override / set additional
configuration options on a per-subnet basis.

Additionally it creates the file /etc/default/dnsmasq.<zone> that
provides default options for the dnsmasq service.

Leases are stored in /var/lib/misc/dnsmasq.<zone>.

Co-Authored-By: Alexandre Derumier <aderumier@odiso.com>
Signed-off-by: Stefan Hanreich <s.hanreich@proxmox.com>