allow setting the ip to listen on

Add new argument 'riak::ip' in order to explicitly set the ip riak services listen on. The previous value of $::ipaddress is rather brittle as it amounts to: ip that's first parsed from ifconfig and doesn't match ^127. 

Old behaviour is preserved by using $::ipaddress as default. 

I was unable to run the spec tests by following the documentation. End result on every test was:

```
Error from DataBinding 'hiera' while looking up 'riak::version': FileSystem implementation expected Pathname, got: 'Hash'
```

Testing in vagrant showed the desired changes with no regressions however.