Fix checking for existence of EC2 tags

Without `.items()`, the comparison is:

```
tagdict=set([u'role', u'Name'])
tags=set([u'role'])
```

With `.items()`:

```
tagdict=[(u'role', u'new-app-server'), (u'Name', u'app-server-13-11-27-09-00')]
tags=[(u'role', u'old-app-server')]
```

See https://github.com/ansible/ansible/issues/5077 for history.