add a way to force clean in resource strategy

[tiboun]

While using clean_and_keep on dataset and on all the tables below it, clean first can't be done after the first run since tables are kept. Different ways to handle this issue :

• add the ability to force clean the resource. This is useful only for dataset. Therefore tables below it don't need to be clean since they will be wiped directly
• add a create_if_not_exists that will allow reuse of an existing resource.

Both of them are interesting options to add.