fix failed IT cases for AD

[ylwu-amzn]

Issue #, if available:

Description of changes:
Fix failed IT cases for AD monitor.

Test with security enabled
./gradlew integTest -Dtests.rest.cluster=localhost:9200 -Dtests.cluster=localhost:9200 -Dtests.clustername=es-integrationtest -Dhttps=true -Duser=admin -Dpassword=admin -Dsecurity=true

Test with security disabled
./gradlew integTest -Dtests.rest.cluster=localhost:9200 -Dtests.cluster=localhost:9200 -Dtests.clustername=es-integrationtest

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov]

Codecov Report

Merging #320 (3c2d877) into master (37d537d) will increase coverage by 0.01%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master     #320      +/-   ##
============================================
+ Coverage     79.32%   79.33%   +0.01%     
+ Complexity      203      202       -1     
============================================
  Files           151      151              
  Lines          5324     5324              
  Branches        701      701              
============================================
+ Hits           4223     4224       +1     
+ Misses          727      725       -2     
- Partials        374      375       +1     

Impacted Files	Coverage Δ	Complexity Δ
...ing/destination/client/DestinationEmailClient.java	72.50% <0.00%> (-5.00%)	7.00% <0.00%> (-1.00%)
...distroforelasticsearch/alerting/core/JobSweeper.kt	72.19% <0.00%> (+0.53%)	35.00% <0.00%> (ø%)
...roforelasticsearch/alerting/alerts/AlertIndices.kt	67.17% <0.00%> (+1.52%)	0.00% <0.00%> (ø%)

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