Add supplementary informations on run-iperf output

[DUFOUR Olivier]

As it is right now, run-iperf output is a little bit mystical if we want to interpret the result quickly.

Currently for each test to one destination, the following is outputted (if https://review.opendev.org/c/openstack/charm-magpie/+/879333 gets merged) :
{
 "src_port": "48188 + 48206 + 48172 + 48198",
 "dest_port": "5001",
 "dest_node": "magpie-pxe-space_14",
 "session": "2 + 3 + 1 + 4",
 "transferred_bytes": 113723834608,
 "bits_per_second": 15158166805,
 "src_ip": "192.168.24.57",
 "dest_ip": "192.168.24.54",
 "time_interval": "0.0-60.0",
 "concurrency": 4,
 "GBytes_transferred": 105.914,
 "Mbits_per_second": 14455,
}

It would be great to get more details for analysis later or simply if we need to do some manual testing/debugging.
* Adding the source interface used for the benchmark
* Adding the MAC addresses for both source and destination, useful for network debugging

Currently proposed output :
{
 "src_port": "48188 + 48206 + 48172 + 48198",
 "dest_port": "5001",
 "dest_node": "magpie-pxe-space_14",
 "session": "2 + 3 + 1 + 4",
 "transferred_bytes": 113723834608,
 "bits_per_second": 15158166805,
 "src_ip": "192.168.24.57",
 "dest_ip": "192.168.24.54",
 "time_interval": "0.0-60.0",
 "concurrency": 4,
 "GBytes_transferred": 105.914,
 "Mbits_per_second": 14455,
 "src_interface": "br0",
 "src_mac": "e4:1d:2d:1b:fd:b1",
 "dest_mac": "ac:1f:6b:2d:ed:84"
}

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.opendev.org/c/openstack/charm-magpie/+/879948

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.opendev.org/c/openstack/charm-magpie/+/879948
Committed: https://opendev.org/openstack/charm-magpie/commit/81deb4177b06c5f6796706bca6b1b1a072aee992
Submitter: "Zuul (22348)"
Branch: master

commit 81deb4177b06c5f6796706bca6b1b1a072aee992
Author: Olivier Dufour-Cuvillier <email address hidden>
Date: Mon Apr 10 17:59:51 2023 +0900

    Display MAC addresses with run-iperf action

    It adds the capability, for troubleshooting usage, to display
    the source interface used and the related MAC addresses when
    running run-iperf action.

    Closes-bug: #2015175
    Change-Id: I30dc5f910fd47dd08451980d51a6094cf00d401f