ESTestCase should be able to configure index settings

[lchi]

Right now ESTestCase.setup_class must be overwritten in order to set index settings (analyzers in my case) on index creation.

It'd be nice to be able to set this via some class variable (maybe cls.index_settings?) without breaking the existing cls.mapping. If you guys think this is a good idea then I'd be happy to open a PR.

Thanks!

[jmizgajski]

You can have a look at my issue about scalable and testable django support and tell me what you think about the solution to your problem I provide there.