Fix bug callback undefined

[lTimeLordl]

No description provided.

[Pierre-Gilles]

Thx ! Ideally, all functions of the Gladys API should begin with callback = callback || function(){};, but sometimes it's not the case. The API should never crash because of these little mistakes, so if you find another functions in "api/services" with this problem, don't hesitate.

Another problem :
Callback response should always begin with an error param :

function test(a, callback){
    ...
    callback(null, 'ok');
} 

test(a, function(err, result){
    if(err) {
        console.log(err);
    }else{

    }
}

But sometimes it's not the case, we should correct that everywhere.