Fixes for Vagrant 1.4.0 that turned config.ssh.private_key_path into an ...

Add support for handing config.ssh.private_key_path as an array. Putty does not support multiple private key files so we attempt to find a valid putty ssh key by appending the '.ppk' extension to the first path in the config.ssh.private_key_path list. 

Setting the config.putty.private_key_path will still override the default key path.