python: Fix build on Fedora 29
    
    On Fedora 29 "python" is not installed by default, so the build fail
    with:
    
        + make -C python
        make: Entering directory '/builddir/build/BUILD/sanlock-3.6.0/python'
        python setup.py build
        make: Leaving directory '/builddir/build/BUILD/sanlock-3.6.0/python'
        BUILDSTDERR: make: python: Command not found
    
    Using python2 fixes this issue.
    
    Signed-off-by: Nir Soffer <nsoffer@redhat.com>