mgr: do not load disabled modules

mgr: do not load disabled modules

an option named "mgr_disabled_modules" is added in this change to
prevent mgr from loading modules listed in this option. because mgr
loads *all* modules found in the configured path, and per
https://tracker.ceph.com/issues/45147, python subinterpreter could hang
when loading numpy, so this behavior practically creates a deadlock
in mgr.

this issue is found when mgr uses python3.8 runtime. in development
environment, it'd be inconvenient to disable the offending mgr module
without changing the source code, even if we can choose to not install
them, for instance, the enduser can workaround this issue by
uninstalling `ceph-mgr-diskprediction-local`.

an option would be useful in this case, so we can add the module to the
list before mgr tries to load it.

as this issue is found with python3.8 + diskprediction_local (numpy), so
this mgr module is disabled by default if mgr is compiled with python3.8
runtime.

Fixes: https://tracker.ceph.com/issues/45147
Signed-off-by: Kefu Chai <kchai@redhat.com>