Stop overriding CONF.default_log_levels per binary

Overriding CONF.default_log_levels means we loose some explicitly set
default values such as 'iso8601=WARN'. Also explicitly overriding
CONF.default_log_levels means that default_log_levels cannot be set in a
config file.

Change-Id: Ib5103b88ec3f9ccd9d516032c6a13dc2fa4bcb5e
Merge "Stop overriding CONF.default_log_levels per binary"