[DX] changing AliasProcessor::setRootName, so to use "Y-m-d-Gis" instead of random string

This is DX PR to improve index naming.

When I am viewing `/_plugin/head/` or any other plugins for ES, I as developer wanna see more verbose index name. 
So instead `<my-index-name>_53e4714c70333`, I'll see `<my-index-name_2014-08-08-94949>` which represent time of index creating, this could be useful for emergency cases when you need switch alias back to old index ASAP.