Use parallel gzip in box creation if available
Use parallel gzip in box creation if available

This can speed up box creation significantly on modern machines.

vagrant-libvirt/vagrant-libvirt,pradels/vagrant-libvirt,electrofelix/vagrant-libvirt,electrofelix/vagrant-libvirt,vagrant-libvirt/vagrant-libvirt,electrofelix/vagrant-libvirt,pradels/vagrant-libvirt,vagrant-libvirt/vagrant-libvirt