Support multiple indices for one detector

[ylwu-amzn]

User can use multiple indices as detector's input. Currently we only support use one index or index pattern on Kibana. We need to support multiple indices on

1.Create/edit detector page
2.Detector detail page
3.Detector list page
4.Dashboard page

[harwinds]

If this can be done, I believe this will open a lot more doors for writing correlation rules between different log sources. We are currently using ELK as a SIEM solution, this feature can be used to enrich a particular index with more information from a different index.