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_rfsim_fembms.xml

@@ -80,7 +80,7 @@
                 <class>Iperf2_Unidir</class>
                 <desc>Iperf2 UDP DL</desc>
                 <id>rfsim4g_ue_fembms</id>
-                <nodes>localhost</nodes>
+                <node>localhost</node>
                 <svr_id>rfsim4g_enb_fembms</svr_id>
                 <svr_node>localhost</svr_node>
                 <iperf_args>-u -t 30 -b 2M</iperf_args>