Update oslo.notifier and always register options

Update oslo.notifier and always register options

This avoids registering multiple times the option if the registering
functions are called multiple path by different code paths. It's
necessary for the default configuration sample generator to have options
only registered once.

The update of oslo.notifier is needed to avoid it using the inexistant
$host option.

Change-Id: If31974d7ef7fdbf85a88e950ff06c60ccbd6c31d
Signed-off-by: Julien Danjou <julien@danjou.info>