Missing `result_renderer='disabled'` for internal datalad command execution

[mih]

This is the case for, at least, the save calls in containers_add and containers_remove. This should disable the internal rendering of results,
because otherwise a user cannot control the verbosity.

When this is addressed, it makes sense to also start using return_type='generator' to avoid the present list-item-accumulation of results of internal command calls.