Azure-pipelines-agent: Git checkout submodule in YAML

[riezebosch]

It's currently not part of the yaml options.

[ericsciple]

This is deployed to some accounts. The deployment will likely reach the rest of the accounts next week.

Docs haven't merged yet. The syntax will be:

- checkout: self
  submodules: true

[riezebosch]

resources:
- repo: self
  submodules: true

❌ .vsts-ci.yml (Line: 3, Col: 3): Unexpected value 'submodules'

[TingluoHuang]

steps:
- checkout: self
  submodules: true

or

resources:
  repositories:
  - repository: self
    checkoutOptions:
      submodules: true

steps:
- checkout: self

[TingluoHuang]

We have created a new repository for all YAML related issues, please move the current issue to there.
https://github.com/Microsoft/azure-pipelines-yaml