Simplify node management in CI

Some CI tasks/commands like Iperf(), Ping() get multiple nodes (via XML
parameter "nodes") to potentially run UEs on different nodes (hosts) at
the same time. However, I argue that this is not good:

- we don't actually use this -- where we specify multiple UEs, it's
  always "localhost localhost..."
- it is inconsistent, as we typically have a single "node", not nodes,
  and there is a possibility to harmonize (see also next commit)
- if we needed it, there would be better ways to achieve the same.
  First, for hardware-based UEs, the ci_infra.yaml can specify different
  UEs. For simulated UEs, it would be feasible to have multiple XML
  steps run in parallel, e.g., in the XML <parallel> tag, which would
  clarify that multiple UEs on different hosts run in parallel
- it reduces code.

Hence, make a single <node> for these XML steps.

ci-scripts/xml_files/container_5g_e1_rfsim.xml

@@ -76,14 +76,14 @@
                 <class>Attach_UE</class>
                 <desc>Attach OAI UE (Wait for IP)</desc>
                 <id>rfsim5g_ue rfsim5g_ue2 rfsim5g_ue3</id>
-                <nodes>localhost localhost localhost</nodes>
+                <node>localhost</node>
         </testCase>
 
         <testCase id="020021">
                 <class>Ping</class>
                 <desc>Ping ext-dn from all NR-UEs</desc>
                 <id>rfsim5g_ue rfsim5g_ue2 rfsim5g_ue3</id>
-                <nodes>localhost localhost localhost</nodes>
+                <node>localhost</node>
                 <svr_id>rfsim5g_5gc_fixed_ip</svr_id>
                 <svr_node>localhost</svr_node>
                 <ping_args>-c 20 -i0.25</ping_args>