Support more features on feature configuration page

[ylwu-amzn]

Problem

Currently we support 5 features by default. But user can update cluster setting opendistro.anomaly_detection.max_anomaly_features to change it. But user can't add more features even they change it to a larger value.
As a workaround, user can use update API to add more features and the AD result page can show feature charts correctly.

Solution

We should read the max_anomaly_features setting, rather than use the hard coded value.