Fix #4034 by adding an option to set pty to false forcibly.

Fix for #4034. Multiple synced_folder using NFS is correctly working with `config.ssh.pty` is false. But `config.ssh.pty` is true, mount on same point in later version of Vagrant.
Therefore I added the option, and I implemented pty regardless of setting to assume pty false forcibly in `shell_expand_guest_path` of Linux.