Elasticsearch test refactorings

Changes:
* abstract connection behavior to ElasticsearchTestCase
* move _remove and _search methods to ElasticsearchTestCase
* remove index in tearDown to avoid test dependency
* use count requests to get count instead of iterating over scroll

Related fixes:
* get_last_doc now runs on empty index correctly with latest ES
* test suite now always connects to the configured ES cluster
* TestElastic.test_stress fixed