ValueError should use '%' instead of ','

The method _load_service_plugins() in /neutron/neutron/managers.py
 throw a ValueError using ',' to connect log and variables.
ValueError do not support using ',' to connect log and variables,
we should use '%' instead of ','

Change-Id: Ic9b3756ac2bc36ddc689a66c078e205e0674f513
Closes-Bug:#1264210
Merge "ValueError should use '%' instead of ','"