[route53 plugin] Improved sample IAM policy

[limmike]

Route 53 now support DNS resource record set permissions. So we can reduce the IAM policy permission scope to only TXT records with the Condition entry as below

{
    "Version": "2012-10-17",
    "Id": "certbot-dns-route53 sample policy",
    "Statement": [
        {
            "Effect": "Allow",
            "Action": [
                "route53:ListHostedZones",
                "route53:GetChange"
            ],
            "Resource": [
                "*"
            ]
        },
        {
            "Effect" : "Allow",
            "Action" : [
                "route53:ChangeResourceRecordSets"
            ],
            "Resource" : [
                "arn:aws:route53:::hostedzone/YOURHOSTEDZONEID"
            ],
            "Condition": {
                "ForAllValues:StringEquals": {
                    "route53:ChangeResourceRecordSetsRecordTypes": [
                        "TXT"
                    ]
                }
            }
        }
    ]
}

Tested working with certbot 0.38.0 and 2.6.0