Ec2 asg vpc zone identifier list

I've changed the type for vpc_zone_identifier from 'str' to 'list' as this makes more sense and is more consistent. I've also changed the example. 

Before:
```yaml
 vpc_zone_identifier: 'subnet-abcd1234,subnet-1a2b3c4d'
```
Now:
```yaml
vpc_zone_identifier:
      - subnet-abcd1234
      - subnet-1a2b3c4d
```

This change may not be backwards compatible; however IMO making ansible syntax more consistent is a win for everyone.