ec2: add support for rendering public IP addresses in network config

[Chad Smith]

cloud-init 20.1 currently renders only local ip address information in the network configuration emitted for attached NICs.

Ec2 instances can be launched and configured with multiple public IP addresses

https://aws.amazon.com/blogs/aws/multiple-ip-addresses-for-ec2-instances-in-a-virtual-private-cloud/

Ec2 IMDS network data exposes public IPv4 addresses in the following metadata keys:
* network.interfaces.macs.<MAC_ADDR>.public-ipv4s = "18.218.219.181"
* and an association of public ip to the private IPs allocated via the key
     "ipv4-associations": {
 "18.218.219.181": "172.31.44.13"
 },

Cloud-init should render known public ipv4 address configuration on the node to avoid network round trips for local services that may connect with the VMs known public IP

[James Falcon (falcojr)]

Tracked in Github Issues as https://github.com/canonical/cloud-init/issues/3622