detected_network.yml: Disallow multi-line var second_gateway_found ~= exclude_devices (to avoid polluting /etc/iiab/iiab.ini)

[holta]

To keep /etc/iiab/iiab.ini from being polluted with multi-line variable artifacts like the following...

exclude_devices = none
wlp12s0
num_lan_interfaces = 1

...which prevented @EMG70 from running Admin Console earlier today, when he had 3 gateways (no doubt without realizing it) apparently including (1) his laptop's Ethernet (2) his laptop's internal WiFi (3) a USB WiFi dongle. As confirmed by his Dell laptop's iiab-network.log (Line 9374) that is broken into 2 lines here:

2022-08-01 07:22:05,597 p=3352 u=root n=ansible | TASK [network : Set exclude_devices if default gateway has been detected for wlx1ca7705e063c
wlp12s0] ***

So this PR should help intercept problematic network attempts of this sort, e.g. when IIAB is being pushed too far into complex networking scenarios we're not ready to support at this time!

A bit of background:

• Future advanced networking improvements #815 (comment)
• Dell inspiron 1750 wlan issues [trying for any hotspot, internal or WiFi USB dongle] #3324
• Live w/ red errors to set can_be_ap ('failed_when: False' was too strong) + WiFi firmware questions w/ Linux Mint (Broadcom or Realtek) #3329 (comment)
• Live w/ red errors to set can_be_ap ('failed_when: False' was too strong) + WiFi firmware questions w/ Linux Mint (Broadcom or Realtek) #3329 (comment)

[holta]

It took hours to get to the bottom of this issue (thank you to @shanti-bhardwa, @jvonau and @tim-moody) but now the root cause is understood.

So this PR should reduce the agony, when this happens to the next person — who inevitably likewise adds a USB WiFi dongle(s) when their laptop already has an Ethernet and WiFi connection to their home router etc — and then runs iiab-network

[holta]

Good enough.

Can be further refined later if @jvonau / others want to refactor / enhance.

[jvonau]

Adding a usb-dongle is not the problem, using the same dongle at an upstream connection before running iiab-network is the problem as there would be not a device available to assign to for use with hostapd. This leave the new device unused within the GUI needs to be better documented, for better user education.