script/gen_static_command_descriptions: add mock for rados python binding

script/gen_static_command_descriptions: add mock for rados python binding

to address issue like

Traceback (most recent call last):
  File "/home/jenkins-build/build/workspace/ceph-pr-docs/doc/scripts/../../src/script/gen_static_command_descriptions.py", line 168, in <module>
    print(json.dumps(gen_commands_dicts(), indent=2, sort_keys=True))
  File "/home/jenkins-build/build/workspace/ceph-pr-docs/doc/scripts/../../src/script/gen_static_command_descriptions.py", line 163, in gen_commands_dicts
    comms = from_mon_commands_h() + from_mgr_modules()
  File "/home/jenkins-build/build/workspace/ceph-pr-docs/doc/scripts/../../src/script/gen_static_command_descriptions.py", line 85, in from_mgr_modules
    comms = sum([list_mgr_module(name) for name in names], [])
  File "/home/jenkins-build/build/workspace/ceph-pr-docs/doc/scripts/../../src/script/gen_static_command_descriptions.py", line 85, in <listcomp>
    comms = sum([list_mgr_module(name) for name in names], [])
  File "/home/jenkins-build/build/workspace/ceph-pr-docs/doc/scripts/../../src/script/gen_static_command_descriptions.py", line 65, in list_mgr_module
    mgr_mod = __import__(m_name, globals(), locals(), [], 0)
  File "/home/jenkins-build/build/workspace/ceph-pr-docs/src/pybind/mgr/volumes/__init__.py", line 2, in <module>
    from .module import Module
  File "/home/jenkins-build/build/workspace/ceph-pr-docs/src/pybind/mgr/volumes/module.py", line 8, in <module>
    from .fs.nfs import NFSCluster, FSExport
  File "/home/jenkins-build/build/workspace/ceph-pr-docs/src/pybind/mgr/volumes/fs/nfs.py", line 6, in <module>
    from rados import TimedOut
ImportError: cannot import name 'TimedOut'

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