Add network::ifcfg custom type to create ifcfg-* files.

It works, but the ifcfg.erb template creates really ugly
files.
- add default value "present" for ensure parameter of ifcfg type
- core class: use create resource to create all supplied network interfaces
  chose the resource type based on the osfamily (only RedHat
  is supported so far)
- require net-tools on RedHat systems