Add index name prefix config option

[icamys]

Sometimes test is killed by OS if it consumes too much memory. In such cases the database is not cleaned up and we should delete test indices manually. It would be nice to add index name prefix config option for situations described above. This prefix will give us ability to delete indices using wildcards.

[roiravhon]

@icamys That's a great idea! you are more than welcome to PR this :)