Added Packer template for vCenter Server Appliance 5.5

My Packer template & setup script for VMware vCenter Service Appliance (VCSA) 5.5.  Works with Vagrant's VMware Fusion provider & GoSDDC's `vagrant-vcenter` provider.  I'm planning on using this to automate the deployment of vCenter simulators.

There were a couple of "gotchas" as you can see in the setup script. The one gotcha not documented in the code itself is where to get VCSA from.  VCSA comes as an OVA and one must use `ovftool` to convert that OVA into VMX format so that it can be fed into Packer's `vmware-vmx` builder.  If there's a better way of doing this please let me know!