chaging type of vpc_zone_identifier from str to 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:
```
vpc_zone_identifier: 'subnet-abcd1234,subnet-1a2b3c4d'
```
Now:
```
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.