make the <dhcp> element optional in network.rng

make the <dhcp> element optional in network.rng

In practice this has always been optional, but the RNG has shown it as
mandatory, and since all the examples for make check had it, it was
never noticed. One of the existing test cases has been changed to
check for this.

I also noticed that the dhcp/host/ip was still defined as <text/>,
but should really be <ref name='ipv4-addr'/>