pybind/mgr/cephadm: fix typing issue

pybind/mgr/cephadm: fix typing issue

Found using mypy 0.990.
Newer mypy versions see the check as tautological because `self.func` is
typed as always being `Callable` and thus always true. I don't know if
the typing is wrong or if the if-block is tautological (and redundant).
For now, silence the error by using `getattr` instead of direct
attribute access.

Signed-off-by: John Mulligan <jmulligan@redhat.com>