Use no explicit key file when `private_key_path` is `:agent`.

Basically, I wanted a Vagrantfile setting to instruct vagrant-multi-putty to use keys from Pageant. This is actually the same effect as `-p`, but in the Vagrantfile so you don't have to specify it all the time.

I considered using a separate setting or somehow turning `nil` into this, but a special symbol seemed like the best solution to me.