SSH RequestTTY Support

Having run into the issue with syncing folders on RHEL-based machines and EC2 instances, and noticing the number of concerns related to this on the issue board, I have attempted to resolve this issue.  I followed 'forward_agent' as a template and as a result may have overdone it on where to put this option.  In particular, I added this to kernel_v1, but wasn't sure if I should probably leave that alone.

My testing is limited to a few Linux distros at present, so if issues are found on other platforms I can take some time to look into that.

RequestTTY can be invoked in the Vagrantfile by using:
  config.ssh.request_tty = true

Also note that that while SSH supports a few choices for this option, I have limited it to two (false = No, true = Force).