Builds the correct set of params depending on whether or not the
identifier is an instanceId or a name.

@param {state} instanceState
@param {string} identifier
@returns {object} params for the ec2.describeInstances call.
function buildParams (identifier, instanceState) {
  var params = {}

  if (instanceState) {
    params.Filters = [
      {
        Name: 'instance-state-name',
        Values: [instanceState]
      }
    ]
  }

  if (isInstanceId(identifier)) {
    params.InstanceIds = [identifier]
    return params
  }

  params.Filters = params.Filters || []

  params.Filters.push({
    Name: 'tag:Name',
    Values: [identifier]
  })

  return params
}