[Feature Request] Send slack notifications to multi workspace.

[kumada626]

What you expected to happen:
Currently, Screwdriver can send slack notification to only single slack workspace.
It is nice that users can receive slack notification on multi workspace.
For example, one job notifies channel A in workspace B, another job notifies channel C in workspace D.

[ibu1224]

@jithine @kumada626 cc: @screwdriver-cd/admin-all

[Breaking Change]
This design is partially backward incompatible.
Please comment if you have any concerns.

YJ has started to work on this issue.

How users specify their workspace

settings:
  slack:
    channels:
      - workspace1:notice
      - workspace2:notice
      - notice

Set workspace name and channel name with ":".
If no workspace is specified, the workspace set by default, described below, is used as the notification settings.
Users have backward compatibility.

I am all about transitionality.
The use of ":" requires a small learning cost to set up. However, there is documentation in Screwdriver.cd, so it is sufficient if it is described there.

Modifications to be Made by the Administrator

1. Enable to place a slack bot token for each workspace

[Breaking Change]
notifications:
  options:
    # Throw error when validation fails (default true); otherwise show warning
    throwValidationErr: true
   slack:
      defaultWorkspace: workspace1
      workspaces:
        - name: workspace1
           token: your-slack-bot-token
        - name: workspace2
           token: your-slack-bot-token

This configuration method is not backward compatible.
Cluster administrators must follow the new configuration method. Until then, no slack notifications will be made.

defaultWorkspace means the workspace to be notified if workspace is not specified.

2. Modification of notification-slack
   Please confirm details at the time of revision. The following is a list of objects to be corrected.

• Modification of joi.schema to read changed settings
• Added new method to switch notification destinations
• Added the ability to use multiple tokens