Use fallback Prometheus port in SLES 16

[antgamdia]

Description

This pull request updates the default port configuration for Prometheus to better support SLES 16 environments, where port 9090 is already used by Cockpit. It also improves the documentation to clarify these changes and the related default values.

Prometheus port default changes:

• Updated the default value of trento_prometheus_port in roles/trento/defaults/main.yml to use port 9191 on SLES 16, while keeping 9090 as the default elsewhere. This prevents port conflicts with Cockpit on SLES 16.
• Updated the documentation in README.adoc to explain the new default behavior for trento_prometheus_port, specifying that it defaults to 9191 on SLES 16 and can be overridden if needed.
• Updated the deprecated prometheus_port variable documentation to reflect the new SLES 16 default (9191), improving clarity for users migrating or deploying on SLES 16.

Fixes #TNRT-4636

How was this tested?

IRL

Documentation changes

Yes

Additional information

Related to #132

[chargio]

suggestion: I would suggest adding a TRENTO_PROMETHEUS_PORT env variable, so that customers can define the best solution there.

thought: And I think more and more than because Prometheus is not supported by Trento and requires additional knowledge to deploy properly, our documentation should specify that. We could then deploy always prometheus in a non-standard port (9191) documenting that is a "test only" solution, and that the customer should deploy their own solution properly in the right port, highlighting the conflict with Cockpit and the reuse of port 9090. That would also avoid problems with customers that expect performance and scalability of the solution to be enough even if that is not part of the solution, and point to MLM as the supported SUSE solution.