Add concurrency option in plan steps

[dtsomp]

Use Case

I have an operating process where I need to run a script on multiple targets running different types of services. Most of them can be done safely in parallel to each other, but one class of targets needs to be done one-by-one. So I would like to have a plan where a step takes care of the first group of servers in parallel and another step where the second target group is handled with concurrency=1 .

Describe the Solution You Would Like

Plan steps should have an optional concurrency argument.

steps:
  - description: Run script in parallel
    script: mymodule/parallel.sh
    targets:
      - server_group1                                                                                                                                                                                                                                                                                                                  
  - description: Run script one-by-one
    script: mymodule/nonconcurrent.sh
    targets:
      - server_group2
    concurrency: 1