Add an option to reindex all when the elasticsearch index does not exist

If the Elasticsearch index has been dropped, it can be interesting to reindex all the content at startup time.
So we don't need a human operation to run the reindex from the admin center.

Here is the new option:

# Reindex the repository content on startup if the index is empty
elastcisearch.reindex.onStartup=false

When true it will reindex on startup and wait for 20s for the indexation to be done,
it it takes more time the indexation continue in background and the application starts with a partial index

Note that the value is false by default because there is no way to know if the instance is new (in this initial case we don't want to reindex)