Detecting OpenVZ environment (hardware node or container)

Add hard classes ***openvz_hardware_node*** and ***openvz_container*** accordingly  if we are in an OpenVZ environment:

On an hardware node: 

    # cf-promises/cf-promises -v | hcgrep openvz
    openvz_hardware_node

Inside an OpenVZ container:

  
    # cf-promises/cf-promises -v | hcgrep openvz
    openvz_container

Tested on version 2.5 of OpenVZ