Code review comment for ~slyon/network-manager:netplan-integration

Thanks!

I've fixed the utf-8 (non-ASCII keyfiles) issue reported by @seb128 and @danilogondolfo as suggested by Danilo (i.e. checking for the "text/plain" mime type instead).

Thanks Sebastien for the "Error: failed to reload connections: GDBus.Error:org.freedesktop.DBus.Error.UnknownMethod" error, that's a good catch! It didn't happen on my Lunar autopkgtests, but I could reproduce it on Mantic, too. Seems to be some kind of race condition when debhelper tries to re-start NetworkManager.service after upgrade and then the postinst script is calling into "nmcli", while NM is not yet fully up. I've added a small "nm-online -s" line in there to make it wait for NM, which now passes the autopkgtests on Mantic again:
```
autopkgtest [15:07:36]: @@@@@@@@@@@@@@@@@@@@ summary
wpa-dhclient PASS
nm.py PASS
killswitches-no-urfkill PASS
urfkill-integration PASS
nm_netplan.py PASS
qemu-system-x86_64: terminating on signal 15 from pid 1944756 (/usr/bin/python3)
```

For the "Warning: there is another connection with the name '...'. Refer to the connection using its uuid '...'" message, I don't really know where it comes from, as we're only modifying connections through nmcli, using their UUID identifiers... Could you try to provide a reproducer for that?

PTAL.