Make state=present the default for ec2_tag

Using `ec2_tag` module without the state argument
currently results in:
```
failed: [127.0.0.1] => {"failed": true, "parsed": false}
```
This fix makes `state=present` the default
Merge pull request #5312 from willthames/ec2_tag_enforce_state

 Make state=present the default for ec2_tag