owut usage: overview.upstream_url reporting local mirror, makes things slow

[efahl]

I've been using the overview.upstream_url from our recent work (https://github.com/openwrt/asu/commit/1fa82886fa28514d90b4beca209d99b05c3c7b80) and the reported local URL (local to sysupgrade server, I assume) is pretty slow at my location (US west coast; traceroute shows mirror-03.infra crossing the Atlantic, so ttl jumps over 150ms, owut slower by near 50%).

@aparcar Should we add another config variable, maybe UPSTREAM_CDN, and report that as the value in overview.json (and nowhere else)? If yes, just let me know and I'll get a PR in.

# where to find the ImageBuilders
UPSTREAM_URL = "https://mirror-03.infra.openwrt.org"  # Local mirror for all internal use.
UPSTREAM_CDN = "https://downloads.openwrt.org"        # CDN for reporting to clients.

Or maybe just change the deployed config.py on sysupgrade.openwrt.org to contain the CDN entry point downloads...? I'm guessing that would probably breaks the webhooks or something???

$ time owut check -v | grep Downloaded
Downloaded https://mirror-03.infra.openwrt.org/snapshots/targets/x86/64/profiles.json to /tmp/owut-platform.json
... 4-5 other files ...
real    0m 6.45s    <<< Typical, varies from 6.4-6.8s
user    0m 0.01s
sys     0m 0.03s

$ time owut check -v | grep Downloaded
Downloaded https://downloads.openwrt.org/snapshots/targets/x86/64/profiles.json to /tmp/owut-platform.json
...
real    0m 3.62s    <<< Varies from 3.4-3.6
user    0m 0.02s
sys     0m 0.02s