Don't translate debug level logs in nova.cells

Our translation policy
(https://wiki.openstack.org/wiki/LoggingStandards#Log_Translation) calls
for not translating debug level logs. This is to help prioritize log
translation. Furthermore translation has a performance overhead, even if
the log isn't used (since nova doesn't support lazy translation yet).

Change-Id: Idb346d04f24cbbc19b6e41af6e4782ac1c49d3b8
Merge "Don't translate debug level logs in nova.cells"