Uses AutoScaling.describeLaunchConfigurations to check existence of the
launch configuration. If the list of LaunchConfigurations returned is empty
doesLaunchConfigurationExist is resolved as False, if it's not empty it is
resolved as True.

@param autoScaling
@param launchConfigurationName

@returns {Promise}
function doesLaunchConfigurationExist (autoScaling, launchConfigurationName) {
  return autoScaling.
    describeLaunchConfigurationsPromised({
      LaunchConfigurationNames: [ launchConfigurationName ]
    })
    .then(function found (data) {
      return !_.isEmpty(data.LaunchConfigurations)
    })
}