Revise template for /etc/netplan to mitigate 2+ min stall during Ubuntu 19.04 boot

[holta]

Fixes #1586's more-than-2-minute delay ("A start job is running for Wait for Network to be Configured") on every boot of IIAB on Ubuntu 19.04

Thanks to @jvonau who agrees with this change suggested by @floydianslips [*] while hoping others test this across Ubuntu 18.04, 19.04 and others (with many different network conditions, internal AP and external AP, etc) in coming months!

[*] originally suggested by https://askubuntu.com/questions/972215/a-start-job-is-running-for-wait-for-network-to-be-configured-ubuntu-server-17-1/1110474#1110474 ?

Related to: #1586, PR #1633

[holta]

@m-anish: this PR relates to the 2 min delay you were experiencing, the general desire for optional: true causing it.

Please review https://github.com/iiab/iiab/wiki/IIAB-Networking#know-your-topology-and-ip-addresses that @jvonau just assisted in writing with me, Thanks!

[holta]

@jvonau & I noticed today that this PR (#1634) was effectively reverted a year later, by:

• PR Ap0+wifi up down rebased [simultaneous WiFi-as-hotspot AND WiFi-as-client] #2380

Is that possibly concerning? In any case, there's a lot of background design discussion here if we need it:

• Outstanding IIAB issues (network & Node-RED) on Ubuntu Server 19.04 [released 2019-04-18] #1586