arm image deploy: don't use HDD_1
    
    Setting HDD_1 to %HDD_2% is broken in recent openQA:
    https://github.com/os-autoinst/openQA/pull/3309#issuecomment-721906935
    I count this as a bug, but we can solve it like this, I think -
    we don't actually need to set this test up so carefully to only
    have the disk image as HDD_1 and no HDD_2, we can actually just
    let the disk image be HDD_2 and have an empty disk as HDD_1 and
    the test still works, qemu will boot from the second disk and we
    can upload it and everything's fine. So let's just go with that.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>