Nuke: render use existing frames with slate offsets the published render - AY-1433

[tokejepsen]

Changelog Description

Due to frameStart data member on representation for existing frames, the frame indexes would be re-numbered when integrating due to this; https://github.com/ynput/OpenPype/blob/develop/openpype/plugins/publish/integrate.py#L712-L726

Removing frameStart had no effect on publishing workflows, local or farm.

Also fixed an issues with slate collection which could misbehave if the instance node had "slate" in the name.

Resolves #5883

Testing notes:

1. Add nuke slate to the script
2. Publish render using local machine rendering
3. Publish render with Use existing frames option (will reuse render from previous step)
4. Check integration logs where it copied files to publish destination, which should keep frame indexes intact. Looks like this:

Copying file ... P:\PROJECTS\toke_testing\OP_7336_slate_workflow_broken_if_existing_frames\work\compositing\renders\nuke\renderCompositingSlate\renderCompositingSlate.0000.exr -> P:\PROJECTS\toke_testing\OP_7336_slate_workflow_broken_if_existing_frames\publish\render\renderCompositingSlate\v031\toke_testing_OP_7336_slate_workflow_broken_if_existing_frames_renderCompositingSlate_v031.0000.exr

[jakubjezek001]

worked like a charm ;) Tested on local render, existed frames and exited on farm. All had the same results. Thanks!