net: support set public ip range for forward mode nat

net: support set public ip range for forward mode nat

Support setting which public ip to use for NAT via attribute
address in subelement <nat> in <forward>:

...
  <forward mode='nat'>
      <address start='1.2.3.4' end='1.2.3.10'/>
  </forward>
...

This will construct an iptables line using:

  '-j SNAT --to-source <start>-<end>'

instead of:

  '-j MASQUERADE'

Signed-off-by: Natanael Copa <ncopa@alpinelinux.org>
Signed-off-by: Laine Stump <laine@laine.org>