Add result index retention for anomaly detection

[ylwu-amzn]

Currently AD just rollover result index if the doc count exceeds 300k or the index age exceeds 30 days. We have no easy way to let user define how long they want to keep the results. Too many AD result indices may bring down the cluster performance.

Possible solutions:

Option1

Leverage Index Management plugin. We can ask user define some retention policy when create detector and set up actions in Index Management plugin

Option2

We build our own retention job to maintain AD result indices and provide user some dynamic settings.

Option1 will avoid reinvention work but we add another dependency on Index Management plugin.
For Option2 , we can add retention logic in AD ES plugin and no more dependency included.