Build: replace ipaplatform magic with symlinks generated by configure
    
    The original approach with __path__ implemented
    by 8f98fa1bd5f1da207fab6f89b75e0cdc19d00797 broke Pylint:
    We decided to resort back to symlinks as it is easiest solution
    which does not break pylint in weird ways.
    
    This commit introduces configure --with-ipaplatform option.
    
    https://fedorahosted.org/freeipa/ticket/6418
    
    Reviewed-By: Lukas Slebodnik <lslebodn@redhat.com>
    Reviewed-By: Stanislav Laznicka <slaznick@redhat.com>
    Reviewed-By: Christian Heimes <cheimes@redhat.com>