fix issues with https tests

BatchRestAPITest/BatchRestAPITransactionTest: skip https tests until https://github.com/neo4j/neo4j/pull/891 has been merged
EmptyGraphTest: make sure a new DB gets started in teardown
Neo4jDatabaseCleaner: deal gracefully if referenceNode is not existing
RestAutoIndexTest: fix test isolation by resetting autoindex setup
RestTestBase: make sure keystore is only initialzed once