Introduce AbstractIndexTests which contains the tests, and IndexIT
and ElasticIndexIT which provide the implementations for Lucene and
Elasticsearch respectively. In ElasticIndexIT, provide configurations
for Elasticsearch versions 2.4. 5.6 and 6.2.
The intial implementation only includes a test for the "index change"
command. Further tests will be added in follow-up commits.
Bug: Issue 9168
Change-Id: I45fab4895962815f737f94976c4ef62be8b082a3
a360ca56fc