Switch /usr/bin/ipa to Python 3
    
    When building RPMs with Python 3 support, /usr/bin/ipa will now
    use Python 3.
    The in-tree ipa command will also run on Python 3.
    
    When building with make install, $(PYTHON) is honored and it will
    still default to Python 2.
    
    Part of the work for https://fedorahosted.org/freeipa/ticket/5638
    
    Reviewed-By: Petr Spacek <pspacek@redhat.com>