In 0.10.4 the external_nodes system was upgraded to allow for modular subsystems to be used to generate the top file data for a highstate run on the master.
The old external_nodes option was deprecated after 0.10.4 and was completely removed in 2014.1.5.
The master tops system contains a number of subsystems that are loaded via the Salt loader interfaces like modules, states, returners, runners, etc.
Using the master_tops option is simple:
master_tops:
ext_nodes: cobbler-external-nodes
for Cobbler or:
master_tops:
reclass:
inventory_base_uri: /etc/reclass
classes_uri: roles
for Reclass.
Current Salt release: 2014.1.5
Docs for previous releases on salt.rtfd.org.