mgr: silence GCC warning

mgr: silence GCC warning

the signature of PyObject_CallMethod() is different in python2 and
python3:

in python2: it is

PyObject* PyObject_CallMethod(PyObject *o, char *method, char *format,
...)

while in python3, it is

PyObject* PyObject_CallMethod(PyObject *obj, const char *name, const
char *format, ...)

so, if we compile mgr with python2, we will have following warning:

warning: ISO C++ forbids converting a string constant to ‘char*’
[-Wwrite-strings]
        (char*)NULL);
                   ^

it'd be simpler if we just const_cast<> the method name string, to
silence the warning just like other places we call
PyObject_CallMethod().

Signed-off-by: Kefu Chai <kchai@redhat.com>