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
Fix auto_schedule_networks to resist DBDuplicateEntry

This exception may happen if API and RPC workers are in different
processes.
Also make minor refactoring of auto_schedule_networks method
to avoid unnecessary db queries.
Add missing unit tests and adjust unit test naming style

Change-Id: I6460744e2cffec0b9f009da071597374d8c027f6
Closes-Bug: #1331456
Merge "Fix auto_schedule_networks to resist DBDuplicateEntry"
Merge "ValueError should use '%' instead of ','"