Find ovftool automatically

Hi @frapposelli

and here is another PR with the previous one #2 + the one commit to search for the ovftool in PATH but also in different installation places. I have collected the pathes with VMware Fusion 6 on Mac, VMware Workstation 9 on Windows and VMware Client Integration Plugin 5.5 on Windows.

Now it is much more easier to run packer and build vagrant-vcloud boxes with just packer + this post-processor in path. The ovftool is search automatically if not in PATH.