known_hosts: manage ssh known_hosts files with ansible.

The known_hosts module lets you add or remove a host from the
known_hosts file. This is useful if you're going to want to use the
git module over ssh, for example. If you have a very large number of
host keys to manage, you will find the template module more useful.

This was pull request 7840 from the old ansible repo, which was
accepted-in-principle but not yet merged. The mailing list thread
reading it is:

https://groups.google.com/forum/#!topic/ansible-devel/_e7H_VT6UJE/discussion