update the contents of `performance_test/WMS` folder

[ale-cristofori]

Within a recent project done for one of our clients, we updated the process of setting up the tests for performance and load, specifically for WMS layers. The updated were captured in a dedicated branch of this repo https://github.com/geosolutions-it/scripts/tree/C257-CAE-2022-SUPPORT and now these changes need backporting to master to update the general load and performance testing set up and report it in the official docs.

The steps we need to do within the work covered in this ticket are:

1. Generalise some parameters and settings that were specific to the client in the original branch https://github.com/geosolutions-it/scripts/tree/C257-CAE-2022-SUPPORT.
2. Cherry pick only the changes that touched the the performance_test/WMS folder.
3. Update the template used to created the docs for the client to refer to the updated master branch of this repo when the changes will be merged.

[afabiani]

@ale-cristofori PR available here