Fixed SSHAuthOptions to compare against something that Twisted won't touch

When testing tronfig, I noticed that every time a reconfiguration was completed that there would be a number of extra, idle SSH connection daemons open equal to the number of nodes inside of the configuration file, even if the node configuration or anything related was untouched (for reference, I was changing `buffer_size`). This led me to wonder if the Node objects were always being discarded in favor of new ones, even if the configuration was the same.

Doing some IPDB investigation, it turned out that the `conch_options.identitys` field was actually getting changed between when a Node was created and when it was used (it was `()`, but changed to `['~/.ssh/id_rsa', '~/.ssh/id_dsa']`). It turns out that Twisted will use some defaults for SSH files if it is given none, and the `SSHAuthOptions` object was getting passed instead of a `ConchOptions` object to a Twisted `SSHClientTransport` (it's actually the Tron `ClientTransport`, but it's a subclass of the Twisted object). When Twisted uses the previously mentioned default SSH files, it changes the value of `identitys` if it was Falsy.

This is simply a fix to make the needed `__eq__` comparison not use attributes that Twisted will potentially modify by making a hidden _config object that simply saves the passed `ssh_config` object given in `from_config`.