Update multi_interface.py to not bail out on metric_cleanup function

Error:

$ sudo gmond -m
Unable to find the metric information for 'procs_blocked'. Possible that the module has not been loaded.

Unable to find the metric information for 'procs_created'. Possible that the module has not been loaded.

Unable to find any metric information for 'softirq_(.+)'. Possible that a module has not been loaded.

AttributeError: 'module' object has no attribute 'metric_cleanup'

Further checking, it revealed that multi_interface.py does not metric_cleanup function defined.