[4.0] Create event hook infrastructure

[scottwulf]

Rebase of PR #952 but only including the new infrastructure class

For usage examples, see:
#952
crowbar/crowbar-ha#171
crowbar/crowbar-openstack#717

There are several cases where we have events that should trigger some
activity in some other part of Crowbar. For instance:

• when the public name of a node is saved, it may impact the endpoint
  of an OpenStack service
• when the public name of the VIP of haproxy is changed, it impacts the
  endpoint of OpenStack services
• when the keystone proposal is applied, we may want to reapply all
  proposals that depend on keystone

What we need for this is the ability to notify about the events in the
rails application and then dispatch the notifications to hooks which
listen for them and decide if some action should be triggered.

The main reason we didn't have this in the past is that we likely don't
want to do that in the foreground of the rails application. But now that
we have delayed_job, we can send the notifications and run the hooks in
the background.

In this commit, we add the simple infrastructure about notifications and
hooks:

• the events are defined with a name and a hash that contains the
  details of the event. The structure of the hash depends on the event.

• a simple dispatcher exists that simply connects the hooks to the
  events.

• the hooks only exist for service objects for the time being; a
  service object simply needs to have a event_hook method to register
  the hook, and will need to filter for the events it cares about. The
  signature of event_hook is as follows:
  def event_hook(role, event, details)

It could be argued that the hooks should be registered for some specific
events (hence moving the filter to the event dispatcher), but it's not
worth the complexity for now.

[Itxaka]

Notification Infrastructure

[Itxaka]

FYI @scottwulf we set the title to [VERSION] TITLE when backporting, so its easier to see :)

[matelakat]

Approved from me