Only create es-river if alias already exists
Only create es-river if alias already exists

The Elasticsearch river pulls logs from the logs_redis machines.

The logs which are stored in the logs_redis machines have
`bulk_index=logs-current` specified by the logstream config file.

So we should only create the river to pull those logs from redis
if the `logs-current` alias exists.

alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet