Currently we don't have os_brick DEBUG log levels in Nova when setting
the service to debug mode.
That happens because Nova is forcefully setting oslo.privsep.daemon
levels to INFO to prevent leaking instance XML details (bug #1784062).
Oslo Privsep now supports per-context debug log levels, so this patch
sets the log level name for its only os_brick privsep context to
"os_brick.privileged" to differentiate it from the service it runs under
which uses the default "oslo_privsep.daemon".
This way even though Nova is still disabling its own privileged calls it
won't affect os-brick privileged calls, allowing us to properly debug
block device attach/detach operations.
Closes-Bug: #1922052
Related-Bug: #1784062
Change-Id: I0de32021eb90ca045845a6c7c7e3d27e52895948
a82b2ffce5