Update the example configuration with the correct :dns_key: setting

This is in relation to hnk@b5bfbe063cd0029fbc60fcc331e91469bb5cb709 and theforeman/theforeman.org#225
It extends the documentation to also list the relevant option (**dns_key** set to **false**) in the documentation example.
Merge pull request #226 from hnk/patch-2

Update the example configuration with the correct :dns_key: setting