Allow a plan to "include" another plan.

[simonvane]

What would you like to be added:
Allow a complete plan to be run at a specific point in another plan.

Why is this needed:
Upgrade processes are often identical to the deploy plan, with the exception of a pre-upgrade step and a post-upgrade step. It would be preferable not to duplicate the deploy plan phases in the upgrade plan.

(I would not expect parameters that trigger the included plan to have any effect on the plan it is included in)

Example:
This is a cut-down code snippet of our operator.yaml that hopefully demonstrates what I mean.

plans:
  deploy:
    phases:
    - name: deploy-init
      steps:
      - name: deploy-database
        tasks:
        - database-migrate
      strategy: serial
    - name: deploy-configs
      steps:
        - name: deploy-config
          tasks:
            - apiserver-config
      strategy: serial
    - name: deploy-apiserver
      steps:
        - name: deploy-apiserver
          tasks:
            - apiserver
      strategy: serial
    - name: deploy-virtualservices
      steps:
        - name: deploy-virtualservices
          tasks:
            - virtualservices
      strategy: serial
    - name: cleanup
      steps:
        - name: cleanup
          tasks:
            - database-migrate-cleanup
      strategy: serial
    strategy: serial
  upgrade:
    phases:
      - name: offline
        steps:
          - name: offline
            tasks:
              - take-app-offline
      - name: backup
        steps:
          - name: backup
            tasks:
              - backup-db              
      - name: upgrade
        includePlan: deploy # upgrade is identical to deploy except that we need to take the application offline, backup the database, and bring the app online afterwards.
      - name: online
        steps:
          - name: online
            tasks:
              - bring-app-online