Remove host from per notification options

Specifying host allows you to set the sender so that messages come from
'identity.%(host)s' however this is something you are far more likely to
want to set up per keystone instance than modify when actually emitting
a notification.

It is also incompatible with the way oslo.messaging expects to be called
where you setup a publisher_id once at startup and then all future
messages will reuse this value.

As there is nothing so far using this parameter remove it.

Change-Id: Ie70c313bcebd67cc8c6a3d25eba95405089bd44a
blueprint: oslo-messaging
Merge "Remove host from per notification options"