Add support to provision ESXi 6.x in Razor

According to the docs, Razor currently only supports ESXi 5.5. I've gotten requests to extend that to ESXi 6.x which has been out for a while. Please consider adding this to your backlog for a future release. Thanks!!

In Scope

• Test out the ESXi 5.5 task against ESXi 6.x.
• Create a hierarchy like the following:
  • "vmware_esxi/6" inherits "vmware_esxi" with no modifications
  • "vmware_esxi/5.5" inherits "vmware_esxi" with 5.5-specific modifications
  • Note that the "vmware_esxi" task should always be the latest version of the task we supply.