Add 'dhcpd' role with ISC DHCP server support

- 'dhcpd' role currently installs and configures ISC DHCP server from
  the Debian repository;

- DHCP server will be configured with basic subnet detected
  automatically by default, if you want to include it in a shared
  network or group, you will need to set 'dhcpd_subnets' variable to []
  in inventory;

- 'dhcpd' role will try to detect network interfaces automatically, this
  can be overriden using 'dhcpd_interfaces' list;
Merge pull request #44 from drybjed/role/dhcpd

Add 'dhcpd' role with ISC DHCP server support