Update rabbitmq_upgrade.py

[rasty94]

Improve the check of maint_flag to be able to nodes different to "rabbit"

SUMMARY

This enhancement improves the verification of maint_flag to allow nodes different from "rabbit". The modification extends the system's functionality to handle a wider variety of node types, increasing the flexibility and applicability of the code in different scenarios. This is particularly useful in environments with heterogeneous node architectures.

[csmart]

@rasty94 thanks for the patch, this looks reasonable to me. Any chance you could create a doc snippet for the change and update the git commit to include the reason and fix for the change (perhaps based on the summary above)? Thanks!