generate rndc.key if it's not present

* configure named to allow operations with this key
install configuration files

* introduce a variable that points to the target directory
* update tasks to use the variable
* create necessary directories
* copy necessary files
* install buildbot.net zone
* update configuration templates to use the variable
* install configuration files using the templates
* add handler to reload configuration upon change
* enable and start named service
move traditional root hinting and rfc related zones to a separate file

* this makes the main config file a bit more manageable
update named.conf.options
make buildbot.nte a master

* IP addresses of secondaries were not needed after all as a particular
  slave would be pulling the zone (which is configured in
  named.conf.zones
address comments

* add a comment for buildbot.net slave's address
* remove documentation for unused option
* add a possibility to enable IPv6