Path infrastructure, Linux from ZVOL, OpenNode

This Pull-Request adds the following:
* Changes to the path infrastructure.
  Make it possible to set paths in vm.conf and have them respected by install-vmrc.sh and install-templates.sh
  Make it possible to specify a separate bhyve dataset in vm.conf

* Changes to vm boot infrastructure:
  Make it possible to boot non-FreeBSD VMs from ZVOLs additionally to images.

* Add OpenNode template.