Constraint allocate-public-ip is not recognized on AWS

[Diko Parvanov]

Trying to bootstrap on AWS EC2 with --constraints allocate-public-ip=false results in the warning below and a public IP address is assigned to the EC2 juju controller instances:

juju bootstrap --constraints allocate-public-ip=false --config /home/ubuntu/cloudinit-userdata.yaml aws
Creating Juju controller "aws-us-east-1" on aws/us-east-1
WARNING unsupported constraints: allocate-public-ip

According to https://juju.is/docs/juju/amazon-ec2 allocate-public-ip is supported with AWS.

[Joseph Phillips (manadart)]

The constraint is explicitly marked in the EC2 logic as unsupported, but we should support it.

This might be simple.

We construct an instance of ec2.RunInstancesInput to pass to runInstances.

This has a member NetworkInterfaces, which is a slice of InstanceNetworkInterfaceSpecification. We do not populate this.

That type has a member AssociatePublicIpAddress. If it is as simple as populating this and otherwise relying on defaults, we're good. If we have to fully construct it once opted in, maybe a little harder.