Improve stability of SimpleRecoveryLocalGatewayTests

Fixed testX and testSingleNodeNoFlush by specifying mapping on index creation instead of using dynamic mapping. Dynamic mapping is updated on the cluster level asynchronously and if mapping changes are not applied to the cluster state before node is closed, these changes are not be available after node restart. While data added in the test is preserved, due to absence of mapping, the test still fails. This is a known issue that we are not planning to fix at the moment.