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.
Merge pull request #4102 from mitchellh/windows-guest-rename-compat

Addressed issues with Windows guest renaming on Win7/8