Allows use of Mistral workflows during deployment steps

Allows use of Mistral workflows during deployment steps

Introduces a general mechanism meant to allow for the execution
of workflows during the deployment steps.

Services can define workflow actions to be triggered during a step
in the newly added service_workflow_tasks section. The syntax is:

  service_workflow_tasks:
    step2:
      - name: my_action_name
        action: std.echo
        input:
          output: 'hello world'

Implements: blueprint tripleo-ceph-ansible
Depends-On: If02799e7457ca017cc119317dfb2db7198a3559f
Depends-On: Ibc5707f9f06266fe84ad1dd91dcb984157871d30
Change-Id: I36a642fbc2076ad9e4a10ffc56d6d16f3ed6f27a