fix nsupdate plugin config to work when bind key is not the same as the ...

...domain name

This was driving me crazy trying to work out why dynamic updates weren't working when used with an existing DNS infrastructure.

Sorry for the crazyness of the changes, it seems my text editor decided to be helpful and remove unused whitespace...

Changes are:
README.asciidoc:218
manifests/init.pp:120 & 487
templates/broker/plugins/dns/nsupdate/nsupdate.conf.erb:3