Addressed issues with Windows guest renaming on Win7/8

Removed dependency upon netdom which is not always available on all Windows versions. This implementation that uses PowerShell and WMI should work on all OS and PowerShell versions.

Fixed another issue where host renames would always happen when the hostname was longer than 15 characters. The COMPUTERNAME environment variable only returns the first 15 characters so we no longer use that to check the current host name.
VirtualBox driver should replace only last occurence of suggested VM name.
Ensure VirtualBox virtual disks are stored in the correct directory on Windows hosts
Ensure VirtualBox virtual disks are stored in the correct directory round 2
Merge pull request #4133 from keiths-osc/windows_virtualbox_vmdk_path_fix

Ensure VirtualBox virtual disks are stored in the correct directory on Windows hosts
Fixed issue 4161

Wrong varaible name was used to try and delete the existing Vagrantfile causing an error.
Merge pull request #4162 from mitchellh/fix-issue-4161

Fixed issue 4161
Merge pull request #4102 from mitchellh/windows-guest-rename-compat

Addressed issues with Windows guest renaming on Win7/8
Mention config.ssh.pty in docs
Merge pull request #4211 from emyl/docs-pty-v2

Mention config.ssh.pty in docs
Update ssh_settings.html.md