Repeated parameters improvements

[NicolasMahe]

Improvement of #679

• Use type casting instead of reflection
• Update array test types from []string to []interface{}
• Update test to use data from JSON rather than go struct. That way, we don't choose any type, but let json.Unmarshal decide.

[antho1404]

looks ok to me

[ilgooz]

We should also support other slice types like []string, []bool, []int and so on. Otherwise Validate() will depend on to get data that preduced by a json.Unmarshal(). Because only unmarshalling produces slices as []interface{}. Either we should document this or support other slices as well.

But if we don't support all slices we'll face the same problem hapenned here. We'll need to manually change types to []interface from other slice types when we work with system services because json.Unmarshal() isn't used there.

[antho1404]

For now we need only to support []interface{}, let's just support that and see if one day we need to support something else we can improve that. Here we just focus on validation of these data that are always from json.
We can add a comment to make sure that there is a different way to do it but for now we don't need it for this feature