Initial node routing plug-in for ActiveMQ

This commit adds the RoutingService class for creating and registering
node routing plug-ins.  This commit also includes a routing plug-in that
publishes routing information updates to ActiveMQ.
Convert node routing plug-in to broker plug-in

Convert the routing plug-in interface from an interface for node plug-ins
into an interface for broker plug-ins.

Adapt the existing routing plug-in that publishes endpoint creation and
deletion events for the new interface.
Rename routing_activemq_engine to activemq_routing_engine

For consistency with other plug-ins, rename routing_activemq_engine.rb
to activemq_routing_engine.rb.
ActiveMQ routing plug-in: fix initialisation

Open the connection to ActiveMQ in the initialize method of
ActiveMQPlugin instead of using a conditional assignment in send_msg.

To open this connection, we need values from Rails.application.config,
which is nil at the time that openshift-origin-routing-activemq.rb is
loaded.  Therefore, this commit also moves the instantiation of
ActiveMQPlugin from openshift-origin-routing-activemq.rb to an
after_initialize callback for ActiveMQRoutingEngine.
Rails.application.config is defined by the time the callback is invoked.
ResultIO#parse_output: replace obscure array index

Using 'NOTIFY_ENDPOINT_XXXXXX'.length to represent the length of the
strings 'NOTIFY_ENDPOINT_CREATE' and 'NOTIFY_ENDPOINT_DELETE' is too
obscure for dmcphers's tastes, so simply use a literal 23.
RoutingService: add :{create,delete}_application

Add :create_application and :delete_application events to the
RoutingService interface.

Use these events in the ActiveMQ routing plug-in to send corresponding
events over ActiveMQ.
Send :create_application before :add_gear

Make sure that RoutingService sends notification for an application's
creation before sending the notification for adding the first gear.
plugins/routing/activemq/README: Fix .conf path

Correct the name of the configuration file that the README file
references.