code: function getInstancesKeys(machineName, versionKey){
  
          let manifest = getManifest();
  
          let machine = manifest.machines[machineName];
          if (!machine) {
              throw new Error("Machine does not exists");
          }
  
          let version = machine.versions[versionKey];
          if (!version) {
              throw new Error("Version does not exists");
          }
  
          return Object.keys(version.instances);
      }
docs: Gets the keys of all of the instances of a version of the machine in the repository
  @method getInstancesKeys
  @param {String} machineName The name of the machine to get the instances's keys
  @param {String} versionKey The key of the version to get the instances's keys
  @returns {Array} An array with all the instance's keys of the the version