Azure PRO image boot time quite long due to snap downloads of core and canonical-livepatch install

[blackboxsw]

cloud-init modules:config stage waits ~74-80 seconds of download time on initial system boot on Azure Ubuntu PRO images. This blocking was is due to ua-auto-attach which spends > 1 minute of it's time downloading core and canonical-livepatch snaps. Details in the original cloud-init bug LP: #1921429

Here are some startup logs that point to the download costs for snap installs:

$ systemd-analyze blame
    1min 51.944s ua-auto-attach.service
         17.921s cloud-init.service

$ snap change 2 --abs-time | grep Download
            Spawn Time                       Completion time
Done    2021-03-31T20:27:14Z  2021-03-31T20:28:32Z  Download snap "canonical-livepatch" (95) from channel "stable"
Done    2021-03-31T20:27:15Z  2021-03-31T20:28:24Z  Download snap "core" (10908) from channel "stable"

Biggest gap in ua client logs appears to block on this "snap install of canonical-livepatch":

2021-03-31 20:27:14,328 - util.py:(509) [DEBUG]: Ran cmd: /usr/bin/snap wait system seed.loaded, rc: 0 stderr: b''
2021-03-31 20:28:37,799 - util.py:(509) [DEBUG]: Ran cmd: /usr/bin/snap install canonical-livepatch, rc: 0 stderr: b''

[blackboxsw]

Filed related request for canonical-livepatch support of core18 vs core snaps https://github.com/CanonicalLtd/livepatch-client/issues/181