Uses AutoScaling.describeAutoScalingGroups to check existence of the
auto scaling group. If the list of AutoScalingGroups returned is empty
doesAutoScalingGroupExist is resolved as False, if it's not empty it is
resolved as True.

@param autoScaling
@param asgName

@returns {Promise}
function doesAutoScalingGroupExist (autoScaling, asgName) {
  return autoScaling
    .describeAutoScalingGroupsPromised({
      AutoScalingGroupNames: [ asgName ]
    })
    .then(function found (data) {
      return !_.isEmpty(data.AutoScalingGroups)
    })
}