feat: enable use of dhcp addresses in networkd

- change ipaddress to be optional
- add dhcp option

modules/networking/templates/networkd-network.erb

@@ -2,6 +2,9 @@
 Name=<%= @title %>
 
 [Network]
+<% if @dhcp == true -%>
+DHCP=yes
+<% else -%>
 <% if @ipaddress && @netmask -%>
 Address=<%= @ipaddress %>/<%= IPAddr.new(@netmask).to_i.to_s(2).count('1') %>
 <% end -%>
@@ -14,6 +17,7 @@ DNS=<%= Array(@dns).join(' ') %>
 <% if @domains -%>
 Domains=<%= Array(@domains).join(' ') %>
 <% end -%>
+<% end -%>
 <% if @bridge and @bridge != true -%>
 Bridge=<%= @bridge %>
 <% end -%>