notifier: do not rely on CONF.host

There's nothing in Oslo declaring an 'host' configuration option, so
relying on it isn't a good practice. Let's use the same default value as
intended, but make it work in any case.

Change-Id: I9c898647e68e35435d1a58f920b6e610f217e4e8
Signed-off-by: Julien Danjou <julien@danjou.info>