Suppress printing username config during initialization

In accordance with 'CT1574: SEC-USR-MESS-2: Log only valid usernames for failed login attempts',
nw-cisco usernames are printed during initialization without knowing whether they are valid.
This behavior should be suppressed.

Make output of username:
DEBUG oslo_service.service [-] ml2_mech_cisco_nexus:10.86.1.128.username = admin {{(pid=1619) log_opt_values /usr/local/lib/python2.7/dist-packages/oslo_config/cfg.py:2898}}

Look like output of password:
DEBUG oslo_service.service [-] ml2_mech_cisco_nexus:10.86.1.128.password = **** {{(pid=1619) log_opt_values /usr/local/lib/python2.7/dist-packages/oslo_config/cfg.py:2898}}