Add validation for domain-name of custom dhcp options used in VPCs

[plkokanov]

How to categorize this issue?

/area ops-productivity
/area quality
/kind bug
/platform aws

What would you like to be added:
Extend the infrastructure config validator so that it also checks if a VPC's dhcp options set domain-name key has the correct value corresponding to the region where the nodes are created. The correct value for the domain-name for the us-east-1 is ec2.internal

Why is this needed:
When users specify a VPC ID in the shoot spec, the referenced VPC can use custom dhcp options. The domain-name value inside these options could be missing which leads to the following problem: kubernetes/cloud-provider-aws#384

An incorrect dhcp options would look like this:

{
    "DhcpOptions": [
        {
            "DhcpConfigurations": [
                {
                    "Key": "domain-name-servers",
                    "Values": [
                        {
                            "Value": "AmazonProvidedDNS"
                        }
                    ]
                }
            ],
            "DhcpOptionsId": "dopt-546be43d",
            "OwnerId": "008447767675",
            "Tags": []
        }
    ]
}

A correct one would look like this:

{
    "DhcpOptions": [
        {
            "DhcpConfigurations": [
                {
                    "Key": "domain-name",
                    "Values": [
                        {
                            "Value": "us-west-2.compute.internal"
                        }
                    ]
                },
                {
                    "Key": "domain-name-servers",
                    "Values": [
                        {
                            "Value": "AmazonProvidedDNS"
                        }
                    ]
                }
            ],
            "DhcpOptionsId": "dopt-546be43d",
            "OwnerId": "008447767675",
            "Tags": []
        }
    ]
}

Additionally, when setting the domain-name key field to some non-default value (e.g. compute.custom) a similar problem appears - the nodes will be created in the cluster with names like ip-10-180-25-59.custom.domain, however the Private IP DNS name in the instance resource on AWS side will still be with a default value - ip-10-180-29-19.eu-west-2.compute.internal and the machines will again fail to join the cluster.

The dhcp options id can be retrieved from the VPC's DhcpOptionsId field.

The validation that we introduce could be only temporary depending on whether/how/if kubernetes/cloud-provider-aws#384 will be fixed