Added tagging support for amazon/ebs AMIs

Add tags to an amazon/ebs AMI.  Example configuration:

```json
{
  "builders": [{
    "type": "amazon-ebs",
    "region": "us-west-1",
    "source_ami": "ami-12345678",
    "security_group_id": "sg-12345678",
    "iam_instance_profile": "my-ami-role",
    "instance_type": "t1.micro",
    "ssh_username": "ec2-user",
    "ssh_timeout": "5m",
    "ami_name": "packer-test",
    "tags": {
      "Name": "My AMI",
      "OS_Version": "CentOS6"
    }
  }]
}
```