mgr/orchestrator: functools.partial doesn't work for methods

mgr/orchestrator: functools.partial doesn't work for methods

Use `partialmethod` instead:

```
def decorator_partial(f):
     return partial(f)

def decorator_lambda(f):
     return lambda *args, **kwargs: f(*args, **kwargs)

class C:
     @decorator_partial
     def f(self, arg): pass

     decorator_lambda
     def g(self, arg): pass

C().f(1)
TypeError: f() missing 1 required positional argument: 'arg'

C().g(1)
None
```

Signed-off-by: Sebastian Wagner <sebastian.wagner@suse.com>