DNS slave server support

Right now the provision_dns.module is geared towards managing a master DNS server. But the DNS system is a redundant architecture and it doesn't make sense to just have one server. So we need to be able to manage slave servers.

The way AlternC.org does it is to have a private (password-protected) listing of all the zones that it fetches through HTTP.

I think we can do better than that, so here's what I propose.

A "dns server" should have a "type": "master" (all servers are masters right now) or "slave". Slave servers would be provisionned remotely (so #366418: 0.4: multiple web server support kicks in here). All that would need to be done over there would be to manage a named.conf with the list of zonefiles to sync with. The config file would look like this:

zone "example.com" {
    type slave;
    allow-query { any; };
    file "example.com";
    masters { 1.2.3.4; };
};

"1.2.3.4" would be the address of another master server. So we would also need, in the slave dbserver nodes, a node reference to the "master" server. That master server would in turn tweak automagically its config to allow zone transfers only to those slave servers.

Coming back to the slave: since it's only one file to be transfered, no PHP need to be installed on the slave DNS server, only one file transfered, preferably over SSH or another secure protocol.