service installers: clean up the inheritance
    
    Instead of delegating handling of some parameters like fstore to the parent
    class, the *Instance installers had the logic copy-pasted in their
    constructors. Some other members were also moved to the Service class and the
    parent class constructors in children were fixed to modern standards of
    initializing parent class in Python.
    
    https://fedorahosted.org/freeipa/ticket/6392
    
    Reviewed-By: Stanislav Laznicka <slaznick@redhat.com>