[RFE] Index subcommand shouldn't require a config file

[rsevilla87]

Is your feature request related to a problem? Please describe.
At the moment the index subcommand requires a configuration file:

https://github.com/cloud-bulldozer/kube-burner/blob/6dd21f142b4f869b7153531e9faefde488616000/cmd/kube-burner/kube-burner.go#L235

This requirement is mainly because there's no way to configure the indexer from this cmd.

Describe the solution you'd like

I think we could add some new flags to this sub-command in order to prevent requiring this configuration file going forward.

Describe alternatives you've considered

Some examples I've came up with:

$ kube-burner index -u http://prometheus.com:9090 -t myToken --es-server=https://elastic.com:9200 --es-index=foobar
$ # or
$ kube-burner index -u http://prometheus.com:9090 -t myToken --local-indexing
$ # or
$ kube-burner index -u http://prometheus.com:9090 -t myToken --tarball