Allow configuring the start of DCHP range

[elprans]

Currently, DHCP, if enabled, uses the entire range of network addresses, sans network, gateway and broadcast. This makes life difficult for scenarios where some static address allocation is needed and is performed outside of Terraform configuration. Help this by allowing the configuration of the DHCP address range via the new range_start_offset attribute on the dhcp block. The default is 2, which is equivalent to the current behavior of skipping the first two addresses.