Add SSH keys management.

This fixes #1; I chosed to use a template and to define variables, if this is not the preferred way, switch it to a bare file copy would be quick.

I picked SSH keys from tau for now, but there are small differences on other servers, for example Taziden SSH key is still present on some servers. I guess I can let you check if it's fine like that.

One flaw here is that it put every key on every server. I can manage to change that if you think this is necessary.

Every variable added there by Benjamin are still here, and obviously it works on authorized_keys2 as we have no other choice right now.