cloudstack datasource broken with NetworkManager

Repeat of LP: [#1718029](https://bugs.launchpad.net/ubuntu/+source/cloud-init/+bug/1718029), but with NetworkManager this time.

Since NetworkManager no longer internally uses dhclient (as of version 1.20), the datasource will fail to parse dhclient leases and will always fall back to the default gateway, since it doesn't support the same leases as dhclient and systemd. This can probably be worked around by a user with NetworkManager's [dhcp=dhclient](https://networkmanager.dev/docs/api/latest/NetworkManager.conf.html) setting for now, as long as dhclient gets shipped with their system.

This is suspected not to be reported yet because NetworkManager isn't used as much anymore on server images, and this won't break all cloudstack implementations.