Retrieve required commands for running THetA with our local bcbio python.

def _get_cmd(cmd):
    """Retrieve required commands for running THetA with our local bcbio python.
    """
    check_cmd = "RunTHetA.py"
    try:
        local_cmd = subprocess.check_output(["which", check_cmd]).strip()
    except subprocess.CalledProcessError:
        return None
    return [sys.executable, "%s/%s" % (os.path.dirname(os.path.realpath(local_cmd)), cmd)]