Use oslo.log library instead of system logging module
Use oslo.log library instead of system logging module

The constants of log levels were added in the 1.8 version of
the oslo.log library. So we can replace all usage of system
logging module with log module from oslo.log.

Change-Id: I2992df0bec6337aefa8a75d4853b132bd134fa42
Related-Bug: 1481370

gandelman-a/neutron-lbaas,gandelman-a/neutron-lbaas,gandelman-a/neutron-lbaas