stack/erp5: also use backup cloudooo by default

We have two public cloudooo instances, https:///cloudooo.erp5.net/ and
https://cloudooo1.erp5.net/ but the software release uses only the first
one. This changes to use both instances by default, in a random order.
This happens for the ERP5 instance and also for the test runner.

This also includes a breaking change: the parameter is renamed from
`cloudooo-url` into `cloudooo-url-list` and instead of being a coma
delimited list of URLs, this is now a proper list of URLs.

Some obsolete code and comments about the old `erp5-cloudooo` partition
that used to part of ERP5 software release have also been removed.

On testing side, ZopeSkinsMixin._setUpClass is changed be made more
robust, by waiting that all activities are processed and using a new
xmlrpc client in every iteration ( to prevent issues like the one from
https://erp5js.nexedi.net/#/test_result_module/20230107-548523A7/10 )