add_host should not be a changed action

The action doesn't actually change anything on a system, so setting
the status to changed is wrong. add_host is much like set_fact in that
regard.

Since changed is False by default, there is no need to explicity set
it, so just create an empty dict for result and add to it from there.
Merge pull request #3343 from j2sol/add_host_changed

add_host should not be a changed action