Add notification/dependency between nodes

Sometimes already deployed node using plugins need to be notified of
the addition/removal of other nodes.

This is the case for managing/monitoring plugins.

For example the fuel-plugin-calamari[1] installs the calamari plugin
on a node with base-os named "calamari" for monitoring ceph-osd nodes.

When a new ceph-osd node is added the calamari host need to
acknowledge the new ceph-osd node by running a custom puppet module.

Currently there is no event/subscription/notification to trigger the
execution of the custom module.

The discussion started on IRC: http://irclog.perlgeek.de/fuel-dev/2015-06-15#i_10751910

[1] https://github.com/dancn/fuel-plugin-calamari will be moved to
https://github.com/stackforge/fuel-plugin-calamari/ in the next future.