iperf3: add config to adjust the duration of the iperf run

the time is passed as a string param and is then converted
into seconds when literals "h" or "m" are found.
So it would accept 2m and would convert it to 120s, for example.

Example:

+cfg-iperf3-time@15+

Change-Id: Iff28816f83670751e9e91de31ec59b1b0ad8fc0d
diff --git a/example/defaults.conf b/example/defaults.conf
index aa1e1dc..0b9136d 100644
--- a/example/defaults.conf
+++ b/example/defaults.conf
@@ -123,3 +123,6 @@
   airplane_t_on_ms: -1
   airplane_t_off_ms: -1
   num_carriers: 1
+
+iperf3cli:
+  time: 60