mgr: Make 3rd parameter optional in ceph_get_module_option

mgr: Make 3rd parameter optional in ceph_get_module_option

Fixes: https://tracker.ceph.com/issues/38766
File "/usr/share/ceph/mgr/mgr_module.py", line 863, in _handle_command
2019-03-15T11:28:31.324 INFO:tasks.ceph.mgr.x.mira117.stderr:    return self.handle_command(inbuf, cmd)
2019-03-15T11:28:31.324 INFO:tasks.ceph.mgr.x.mira117.stderr:  File "/usr/share/ceph/mgr/selftest/module.py", line 114, in handle_command
2019-03-15T11:28:31.324 INFO:tasks.ceph.mgr.x.mira117.stderr:    self._self_test()
2019-03-15T11:28:31.325 INFO:tasks.ceph.mgr.x.mira117.stderr:  File "/usr/share/ceph/mgr/selftest/module.py", line 213, in _self_test
2019-03-15T11:28:31.325 INFO:tasks.ceph.mgr.x.mira117.stderr:    self._self_test_config()
2019-03-15T11:28:31.325 INFO:tasks.ceph.mgr.x.mira117.stderr:  File "/usr/share/ceph/mgr/selftest/module.py", line 313, in _self_test_config
2019-03-15T11:28:31.325 INFO:tasks.ceph.mgr.x.mira117.stderr:    assert self.get_module_option_ex("foo", "bar") is None
2019-03-15T11:28:31.325 INFO:tasks.ceph.mgr.x.mira117.stderr:  File "/usr/share/ceph/mgr/mgr_module.py", line 942, in get_module_option_ex
2019-03-15T11:28:31.325 INFO:tasks.ceph.mgr.x.mira117.stderr:    r = self._ceph_get_module_option(module, key)
2019-03-15T11:28:31.325 INFO:tasks.ceph.mgr.x.mira117.stderr:TypeError: ceph_get_module_option() takes exactly 3 arguments (2 given)

Signed-off-by: Volker Theile <vtheile@suse.com>