Nova should be in charge of its log defaults

Oslo's log module has a set_defaults which Nova can use
to set the context logging format and default log levels.
We should not have to fix default levels in oslo and then
sync across to Nova. This becomes all the more important
when we move to oslo.log library so we should get started
now

Change-Id: Id47099d5a755bc2923b05fe4bef270a7c0f99389
Merge "Nova should be in charge of its log defaults"