Create a RulesAction to publish messages to our Camel Broadcaster

[dannylamb]

Depends on #448, #466, and #467

We need a rules action that will accept the message from our EventGenerator actions and allow users to enter multiple queues into a UI for broadcasting. The annotation will look something like this:

/**
 * Provides an action to generate a serialized Update event.
 *
 * @RulesAction(
 *   id = "islandora_message_broadcaster",
 *   label = @Translation("Broadcast Event"),
 *   category = @Translation("Islandora"),
 *   context = {
 *     "message" = @ContextDefinition("string",
 *       label = @Translation("Event message")
 *     ),
 *     "recipients" = @ContextDefinition("string",
 *       label = @Translation("Message Recipients"),
 *       description = @Translation("Queues/Topics to receive the message"),
 *       multiple = TRUE
 *     ),
 *   }
 * )
 */

Combine the array received by recipients into a comma separated list, and insert it as the IslandoraBroadcastRecipients header. Using the publishing service from #467, you should publish the event to the Camel broadcaster, whose queue (islandora-connector-broadcast by default) should be retrieved from configuration. So you'll need to add that to the islandora module's config form.

As far as tests are concerned, we cannot do a full IT test since Alpaca is not on the Travis environment, but we should still test that a message gets published to islandora-connector-broadcast and has the correct headers/message.

[dannylamb]

@Islandora-CLAW/committers #31 is up to resolve this issue.