Add slack notification

[ananthdurai]

-- Addresses the issue: #625.

Desc:
The self-healing and anomaly detected is the crown feature of Cruise Control, but often the actions taken by Cruise Control provide less visibility to its operation. Adding a slack notification for all the anomaly detection and self-healing will increase Cruise control visibility.

Changelog:
The SlackSelfHealingNotifier extends SelfHealingNotifier and merely override the alerts method to send the notification to Slack.

Configuration:
The following property requires to be configured on cruisecontrol.properties

1. slack.self.healing.notifier.webhook (Required)
   (Slack webhook URL) Ref:https://api.slack.com/incoming-webhooks
2. slack.self.healing.notifier.channel (Required)
   (Slack channel name to send the alert, (e.g) #alerts-cruise-control
3. slack.self.healing.notifier.user (Optional)
   User name to display in the slack notification message. (default: Cruise Control)
4. slack.self.healing.notifier.icon (Optional)
   Icon to display in the slack notification message. (default: ℹ️ )

To enable Slack notification, set
anomaly.notifier.class=com.linkedin.kafka.cruisecontrol.detector.notifier.SlackSelfHealingNotifier

[efeg]

Thanks for the patch -- LGTM!

[efeg]

Moved the documentation to Wiki -> https://github.com/linkedin/cruise-control/wiki/Configure-Slack-notifications