fix: avoid class name clash

A new merge request asasked here:
https://github.com/cfengine/copbl/pull/25

The problem appended in this case:
if set_variable_values is used two time in a row in the same bundle
  and
    if the same key name is used

For example, in this case NM_CONTROLLED_in_file was defined for the first
file and the key was ignored for the second file.

"ifcfgeth0[BRIDGE]" string => "br0";
"ifcfgeth0[NM_CONTROLLED]" string => "no";

"ifcfgbr0[NM_CONTROLLED]" string => "no";

files:
    "/etc/sysconfig/network-scripts/ifcfg-eth0"
      edit_line => set_variable_values("app_srvkvm_set_up_bridge.ifcfgeth0");

files:
    "/etc/sysconfig/network-scripts/ifcfg-br0"
      edit_line => set_variable_values("app_srvkvm_set_up_bridge.ifcfgbr0");