amazon ec2: Make fetch-ec2-data more robust

curl does not retry if it is unable to connect to the metadata server.
For some reason, when creating a new AMI with a recent nixpkgs, the
metadata server would not be available when fetch-ec2-data ran. Switching
to wget that can retry even on TCP connection errors solved this problem.

I also made the fetch-ec2-data depend on ip-up.target, to get it to start
a bit later.