[6.15.z] Adapt to changes in broker stderr handling (#15502)

Co-authored-by: dosas <[email protected]>

robottelo/host_helpers/satellite_mixins.py

@@ -246,7 +246,7 @@ def available_capsule_port(self):
             f"ss -tnaH sport ge {port_pool[0]} sport le {port_pool[-1]}"
             " | awk '{n=split($4, p, \":\"); print p[n]}' | sort -u"
         )
-        if ss_cmd.stderr[1]:
+        if ss_cmd.stderr:
             raise CapsuleTunnelError(
                 f'Failed to create ssh tunnel: Error getting port status: {ss_cmd.stderr}'
             )