[Merge] ~slyon/network-manager:netplan-integration into network-manager:ubuntu/master

[Lukas Märdian]

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.
-- 
https://code.launchpad.net/~slyon/network-manager/+git/network-manager/+merge/423735
Your team Network-manager is requested to review the proposed merge of ~slyon/network-manager:netplan-integration into network-manager:ubuntu/master.