I.e. instead of this:

# Default
hibernate.search.backend.index_defaults.foo = bar
# Override for index "index1"
hibernate.search.backend.indexes.index1.foo = bar2

Do this:

# Default
hibernate.search.backend.foo = bar
# Override for index "index1"
hibernate.search.backend.indexes.index1.foo = bar2

Reasons:

1. This structure is a little less verbose.
2. With this structure, we can easily turn backend-level settings into index-level settings ( HSEARCH-3636 In Progress ).
3. This "cascading" structure can be extended to apply to shards, too: shard settings default to index settings, which default to backend settings. This will be useful for HSEARCH-3636 In Progress in particular.