Switched multi-threads testing from iperf3 to iperf

Switched multi-threads testing from iperf3 to iperf because the
iperf3 was not designed for the multiple threads, see [1] and [2].
So, iperf (v2) will be used for measuring tests with -P option.

Made the number of threads as an configurable option (10 by default).

* installing iperf package
* allowing 5001 port
* starting iperf as well as iperf3 at VMs
* added 'multiple_threads_number' option (10 by default)
* added details which tool was used (iperf3 or iperf) in the final table
* changed the default image from Ubuntu18 to Ubuntu20

[1] https: //fasterdata.es.net/performance-testing/network-troubleshooting-tools/iperf/multi-stream-iperf3/
[2] https: //github.com/esnet/iperf/issues/289

Related-PROD: PROD-36943
Change-Id: I39d2c44723344c87836bf7b5fa02b546a08f2ca5
diff --git a/utils/os_client.py b/utils/os_client.py
index e22ce19..63f9151 100644
--- a/utils/os_client.py
+++ b/utils/os_client.py
@@ -310,6 +310,13 @@
                     'cidr': '0.0.0.0/0',
                 },
                 {
+                    # iperf
+                    'ip_protocol': 'tcp',
+                    'from_port': 5001,
+                    'to_port': 5001,
+                    'cidr': '0.0.0.0/0',
+                },
+                {
                     # ping
                     'ip_protocol': 'icmp',
                     'from_port': -1,