Add multi node integration testing into CI workflow

[weicongs-amazon]

Issue #, if available:
#194
Description of changes:
Add multi node integration testing into CI workflow

Testing:

• Verified in desktop with ./gradlew integTest -PnumNodes=3 3 ES nodes are running during the testing
• The workflow works well: https://github.com/opendistro-for-elasticsearch/anomaly-detection/runs/1448785594?check_suite_focus=true
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[codecov]

Codecov Report

Merging #318 (a4fde9a) into master (b270d7e) will increase coverage by 0.05%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master     #318      +/-   ##
============================================
+ Coverage     72.13%   72.18%   +0.05%     
- Complexity     1989     1991       +2     
============================================
  Files           201      201              
  Lines          9570     9570              
  Branches        850      850              
============================================
+ Hits           6903     6908       +5     
+ Misses         2253     2249       -4     
+ Partials        414      413       -1     

Flag	Coverage Δ	Complexity Δ
plugin	71.60% <ø> (+0.05%)	0.00 <ø> (ø)

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

Impacted Files	Coverage Δ	Complexity Δ
...sticsearch/ad/indices/AnomalyDetectionIndices.java	49.34% <0.00%> (+0.32%)	40.00% <0.00%> (+1.00%)
...ransport/DeleteAnomalyDetectorTransportAction.java	56.66% <0.00%> (+4.44%)	16.00% <0.00%> (+1.00%)