CI: use svr_id for Ping in RFsim-based tests

This is not really used as of now, because most IPs are hardcoded. See
one of the next commits for the use of this.

To make sure that we have the correct IP, there is some code that
verifies that a hardcoded IP and the CN IP would be the same.

Note: need two separate ci_infra.yaml entries, because
- for FlexRIC, the container name is different (is the tutorial CN)
- for RFsim5G E1, we don't have ext-dn => ping UPF

ci-scripts/xml_files/container_5g_e1_rfsim.xml

@@ -88,6 +88,8 @@
                 <desc>Ping ext-dn from all NR-UEs</desc>
                 <id>rfsim5g_ue rfsim5g_ue2 rfsim5g_ue3</id>
                 <nodes>cacofonix cacofonix cacofonix</nodes>
+                <svr_id>rfsim5g_5gc_fixed_ip</svr_id>
+                <svr_node>cacofonix</svr_node>
                 <ping_args> -c 20 12.1.1.1 -i0.25</ping_args>
                 <ping_packetloss_threshold>5</ping_packetloss_threshold>
         </testCase>