Feature/riak_test flavor

By adding flavor setting to riak_test config, generic test cases which depends only on S3 API and does not on riak cs / riak internal settings can be run in different settings.

To specify a flaver, riak_test.config should be 

```
{cs, [
    [snip]
     {flavor, FlavorYouWant}
    ]}.
```

Allmost all commit but one is the same as #870 (this PR in on #870 but target to `develop`).