Add folder_iam field to Atlantis config

[nicolst]

Add a folder_iam field to the atlantis config, so we can grant arbitrary roles on folders to each atlantis.

Usage:

atlantis:
  folder_iam:
    - folder: "123456"
      role: roles/rolename

Implemented as a list of objects instead of a map of folder names to lists of roles, because jsonschema is ...................

NOTE: there are project_iam and bucket_iam fields in use in the atlantis block today which are not in the schema, probably because the validation action wasn't added at the time.

[johnksv]

This make sense.
You mentioned project_iam and bucket_iam which are in use today(?). Should these be added to the schema as well to reflect the correct options. Either in this PR or another

[nicolst]

Perhaps, but the problem is that they are maps of strings to list of strings... Which we apparently can't make work in jsonschema. But maybe we could convert the existing use to lists of objects