Fix remote host has disconnected problem

After installing vagrant-sshfs and configuring sshfs paths in a Vagrantfile, if you run:

    $ vagrant up
    $ vagrant destroy
    $ vagrant up

The sshfs mounts no longer work due to the the known_hosts entry for the original Vagrant box that was destroyed.

By adding a few options to the sshfs command to pass to ssh, we can disregard the known_hosts file.

This also eliminates the prompt for accepting the host identity when running `vagrant up` for the first time.