use public ip assignment instead of deprecated ipv6 and private instance flags

[quentinalbertone]

Description

Create instance: merge --ipv6 and --private-instance -> --public-ip (none|inet4|dual)
Create sks: replace --nodepool-ipv6 by --nodepool-public-ip (inet4|dual)
Create sks nodepool: replace --ipv6 by --public-ip (inet4|dual)

Checklist

(For exoscale contributors)

• Changelog updated (under Unreleased block)
• Testing

Testing

$> go run github.com/exoscale/cli compute instance create test --public-ip-assignment none
$> go run github.com/exoscale/cli compute instance create test --public-ip-assignment inet4
$> go run github.com/exoscale/cli compute instance create test --public-ip-assignment dual
$> go run github.com/exoscale/cli compute instance create test

$> go run github.com/exoscale/cli compute sks create cluster-test --nodepool-name test-inet4 --nodepool-size 1
$> go run github.com/exoscale/cli compute sks create cluster-test --nodepool-name test-dual --nodepool-size 1 --nodepool-public-ip dual

$> go run github.com/exoscale/cli compute sks nodepool add <cluster-id> nodepool-dual --public-ip dual
$> go run github.com/exoscale/cli compute sks nodepool add <cluster-id> nodepool-inet4