manager: assume DHCP if we see a lease on taking over an initramfs connection

In general, we aren't really able to tell, but when we see a lease file we're pretty sure that DHCP is what was going on.

https://bugzilla.redhat.com/show_bug.cgi?id=1771792

Fixes: f2fe6c03 ('manager: don't treat the initramfs-configured DHCP connections as generated')