Azure nested management group support

[JS-Jake]

This change allows custodian instances using Azure Functions to access subscriptions inside nested management groups. eg:

• Parent Management Group
  • Management Group 1
    • Subscription 1
  • Management Group 2
    • Management Group 3
      • Subscription 2

[stefangordon]

This is great. There is at least one test in c7n_azure/tests_azure/test_azure_utils.py that will have to be updated. (test_managed_group_helper)

If it is just that one test you won't have to mess with VCR recordings because it patches the API rather than using HTTP playback. You'll just need to update the patch attribute and fix the test.

FYI @logachev I think he worked on this originally.

[kapilt]

@JS-Jake Thanks for the pull request! If you don't mind could you sign the CLA per the developer docs? Direct link here: https://docs.google.com/forms/d/e/1FAIpQLSfwtl1s6KmpLhCY6CjiY8nFZshDwf_wrmNYx1ahpsNFXXmHKw/viewform

[JS-Jake]

@stefangordon Thank you, I'll take a look at that but might need a little help
@kapilt All done :)

[kapilt]

@JS-Jake I went ahead and updated the test, thanks again for the pull request.

[JS-Jake]

@kapilt Awesome, thanks for the help

[kapilt]

lgtm, thanks