Rename Index Default initialization server configuration properties

[danotorrey]

The capability to initialize index set defaults on new Graylog server installations to specific values was introduced in #13018. As discussed in this comment, we think it would be appropriate to rename/add a prefix to the respective server configuration properties to indicate that they are being used as initialization defaults for the first startup only.

A more general rename of a superset of elasticsearch_ properties is being discussed here. This prefix rename should be coordinated with any general rename, since the config library fallbackPropertyName capability only allows us the ability to specify a single fallback property (Graylog2/JadConfig#113).

Full list of index set default initialization properties:

• elasticsearch_analyzer
• elasticsearch_shards
• elasticsearch_replicas
• disable_index_optimization
• index_optimization_max_num_segments
• index_field_type_periodical_full_refresh_interval
• rotation_strategy
• elasticsearch_max_docs_per_index
• elasticsearch_max_size_per_index
• elasticsearch_max_time_per_index
• retention_strategy
• elasticsearch_max_number_of_indices

One potential prefix discussed with @boosty is index_init_default_. Another option could be index_set_init_default_. Perhaps the elasticsearch_ portion prefix is not necessary.