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_flexric_rfsim.xml

@@ -121,7 +121,7 @@
                 <class>Ping</class>
                 <desc>Ping ext-dn from both UEs</desc>
                 <id>rfsim5g_ue rfsim5g_ue2</id>
-                <nodes>carabe carabe</nodes>
+                <node>carabe</node>
                 <svr_id>oai_ext_dn</svr_id>
                 <svr_node>carabe</svr_node>
                 <ping_args>-c 20</ping_args>
@@ -132,14 +132,14 @@
                 <class>Attach_UE</class>
                 <desc>Attach OAI UE 1 and 2</desc>
                 <id>rfsim5g_ue rfsim5g_ue2</id>
-                <nodes>carabe carabe</nodes>
+                <node>carabe</node>
         </testCase>
 
         <testCase id="444444">
                 <class>Detach_UE</class>
                 <desc>Detach OAI UE 1 and 2</desc>
                 <id>rfsim5g_ue rfsim5g_ue2</id>
-                <nodes>carabe carabe</nodes>
+                <node>carabe</node>
         </testCase>
 
         <testCase id="100001">