Fix Base inheritance

A fix, so that this works as expected:

# irb
>> require 'cobbler'
=> true
>> cb = Cobbler::Base
=> Cobbler::Base
>> cb.hostname = 'cobbler.example.com'
=> "cobbler.example.com"
>> d = Cobbler::Distro
=> Cobbler::Distro
>> d.hostname
=> "cobbler.example.com"

Without this patch d.hostname is nil.