validate detector only when start detector; fix flaky test case

[ylwu-amzn]

Issue #, if available:

Description of changes:

1. Only validate feature when start detector
2. Fix flaky test case

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[codecov]

Codecov Report

Merging #377 (a532ead) into master (b40e12a) will increase coverage by 0.27%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master     #377      +/-   ##
============================================
+ Coverage     78.81%   79.09%   +0.27%     
- Complexity     2648     2661      +13     
============================================
  Files           247      247              
  Lines         11723    11726       +3     
  Branches       1009     1009              
============================================
+ Hits           9240     9275      +35     
+ Misses         2002     1971      -31     
+ Partials        481      480       -1     

Flag	Coverage Δ	Complexity Δ
plugin	79.08% <100.00%> (+0.29%)	0.00 <9.00> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ	Complexity Δ
...ndistroforelasticsearch/ad/task/ADTaskManager.java	72.82% <100.00%> (+0.18%)	81.00 <9.00> (+2.00)
...sticsearch/ad/indices/AnomalyDetectionIndices.java	51.26% <0.00%> (+0.31%)	45.00% <0.00%> (+1.00%)
...troforelasticsearch/ad/task/ADBatchTaskRunner.java	88.69% <0.00%> (+0.86%)	64.00% <0.00%> (+1.00%)
...asticsearch/ad/cluster/ADClusterEventListener.java	94.00% <0.00%> (+2.00%)	15.00% <0.00%> (+1.00%)
...pendistroforelasticsearch/ad/cluster/HashRing.java	84.21% <0.00%> (+5.26%)	11.00% <0.00%> (+1.00%)
...ransport/DeleteAnomalyDetectorTransportAction.java	48.38% <0.00%> (+26.88%)	11.00% <0.00%> (+7.00%)

[ohltyler]

Thanks for the fix!

[saratvemulapalli]

Thanks Yaliang for the change and the IT test to cover this.