Use existing host entry

For me whenever I try to do vagrant up on an existing box it results in
an error of the type:

Call to virNetworkUpdate failed: Requested operation is not valid: there
is an existing dhcp host entry in network 'xxx' that matches
"<host mac='xxx' name='xxx'
ip='xxx'/>" (Libvirt::Error)

This probably has something to do with driver.rb:174 and whatever is in
the old_def variable

hosts.each do |host|
	if old_def.already_exist_host?(host)
		update_command = Libvirt::Network::NETWORK_UPDATE_COMMAND_MODIFY
	end
end

it seems that for some reason the update_command assignment in the if
statement never happens. Tried it on two machines, both Debian Jessie
with a mutated CoreOs VirtualBox vm.

This commit forces the NETWORK_UPDATE_COMMAND_MODIFY to happen, it
probably breaks something with the network creation of new boxes but it
at least enables me to do 'vagrant up' on existing boxes.