Invenio-indexer

- indexer: Allow the definition of indices in index_templates, instead of mappings

- bulk: make configurable the number of records to be bulk indexed per task

- cli: add queue selection options
- tests: remove redis as message backend

- Allow passing message queue producer publish arguments via the ``RecordIndexer``
constructor and the ``INDEXER_MQ_PUBLISH_KWARGS`` config.

- Change `schema_to_index` to return only the index and not a tuple with index and
doc type.

- Add support to OpenSearch v2
- Remove `doc_type` param
- Change `record_to_index` to return only the index and not a tuple with index and
doc type.
- Remove the config var `INDEXER_DEFAULT_DOC_TYPE`