Send Slack notifications when a change in build status occurs

[philipjscott]

Context

Currently Screwdriver Slack notifications are sent whenever a build returns a status specified in the user's screwdriver.yaml:

settings:
        slack: 
            channels: 
               - channel_A
               - channel_B
           statuses:
               - SUCCESS
               - FAILURE
               - ABORTED
               - QUEUED
               - RUNNING

It would be convenient if users could configure screwdriver.yaml to send a notification whenever a change in build status occurs.

Objective

Add a feature that allows users to get notifications based on a change in build status by modifying screwdriver.yaml.

Suggestion: Users can pass a list of statuses; Screwdriver will keep track of the most recent occurrence of the statuses listed in delta. Each time the build updates its status, we check:

1. Whether the new status is different from the most recent occurrence
2. The new status is listed in delta

If both conditions are true, the user will get a notification. Example:

settings:
        slack: 
            channels: 
               - channel_A
               - channel_B
           delta: 
               - SUCCESS
               - FAILURE
               - ABORTED

Consider the following status changes:

RUNNING -> FAILURE -> RUNNING -> FAILURE -> RUNNING -> FAILURE -> RUNNING -> ABORTED -> RUNNING -> SUCCESS -> RUNNING -> SUCCESS

The user would get notifications in this order: FAILURE -> ABORTED -> SUCCESS