Use pytest conftest.py and drop pytest.ini
    
    Let's replace some ugly hacks with proper pytest conftest.py hooks.
    Test initialization of ipalib.api is now handled in
    pytest_cmdline_main(). Pytest plugins, markers and ignores are also
    moved into conftest.py. Additional guards make it possible to run tests
    without ipaserver installed.
    
    I added confcutdir to ensure that pytest does not leave our project
    space. Pytest used pytest.ini or setup.py before but pytest.ini is gone.
    
    Signed-off-by: Christian Heimes <cheimes@redhat.com>
    Reviewed-By: Milan Kubik <mkubik@redhat.com>