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_4g_l2sim_tdd.xml

@@ -88,14 +88,14 @@
                 <class>Attach_UE</class>
                 <desc>Attach OAI UE</desc>
                 <id>l2sim4g_ue</id>
-                <nodes>localhost</nodes>
+                <node>localhost</node>
         </testCase>
 
         <testCase id="020001">
                 <class>Ping</class>
                 <desc>Ping trf-gen from LTE-UE 1</desc>
                 <id>l2sim4g_ue</id>
-                <nodes>localhost</nodes>
+                <node>localhost</node>
                 <svr_id>l2sim4g_ext_dn</svr_id>
                 <svr_node>localhost</svr_node>
                 <ping_args>-c 20</ping_args>
@@ -106,7 +106,7 @@
                 <class>Ping</class>
                 <desc>Ping LTE-UE 1 from trf-gen</desc>
                 <id>l2sim4g_ext_dn</id>
-                <nodes>localhost</nodes>
+                <node>localhost</node>
                 <svr_id>l2sim4g_ue</svr_id>
                 <svr_node>localhost</svr_node>
                 <ping_args>-c 20</ping_args>
@@ -117,7 +117,7 @@
                 <class>Iperf</class>
                 <desc>Iperf UDP Downlink</desc>
                 <id>l2sim4g_ue</id>
-                <nodes>localhost</nodes>
+                <node>localhost</node>
                 <svr_id>l2sim4g_ext_dn</svr_id>
                 <svr_node>localhost</svr_node>
                 <iperf_args>-u -t 30 -b 1M -R</iperf_args>
@@ -129,7 +129,7 @@
                 <class>Iperf</class>
                 <desc>Iperf UDP Uplink</desc>
                 <id>l2sim4g_ue</id>
-                <nodes>localhost</nodes>
+                <node>localhost</node>
                 <svr_id>l2sim4g_ext_dn</svr_id>
                 <svr_node>localhost</svr_node>
                 <iperf_args>-u -t 30 -b 3M</iperf_args>