Adding role based filtering for Preview API

[saratvemulapalli]

*Issue #195 *

Description of changes:
Added backend role based filtering for Preview API.
Added new integration tests to verify the behavior.

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[codecov]

Codecov Report

Merging #356 (6d1a85d) into master (dfca231) will increase coverage by 0.10%.
The diff coverage is 76.92%.

@@             Coverage Diff              @@
##             master     #356      +/-   ##
============================================
+ Coverage     77.22%   77.32%   +0.10%     
- Complexity     2277     2281       +4     
============================================
  Files           214      214              
  Lines         10242    10255      +13     
  Branches        909      908       -1     
============================================
+ Hits           7909     7930      +21     
+ Misses         1914     1909       -5     
+ Partials        419      416       -3     

Flag	Coverage Δ	Complexity Δ
cli	79.27% <ø> (ø)	0.00 <ø> (ø)
plugin	77.18% <76.92%> (+0.11%)	0.00 <2.00> (ø)

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Impacted Files	Coverage Δ	Complexity Δ
...ansport/PreviewAnomalyDetectorTransportAction.java	82.43% <76.92%> (-4.46%)	13.00 <2.00> (+2.00)	⬇️
...sticsearch/ad/indices/AnomalyDetectionIndices.java	49.01% <0.00%> (-0.33%)	39.00% <0.00%> (-1.00%)
...opendistroforelasticsearch/ad/util/ParseUtils.java	52.05% <0.00%> (+1.36%)	35.00% <0.00%> (+1.00%)
...asticsearch/ad/cluster/ADClusterEventListener.java	94.00% <0.00%> (+4.00%)	15.00% <0.00%> (+1.00%)
...stroforelasticsearch/ad/AnomalyDetectorRunner.java	46.25% <0.00%> (+6.25%)	7.00% <0.00%> (+1.00%)
...port/SearchAnomalyDetectorInfoTransportAction.java	63.63% <0.00%> (+9.09%)	4.00% <0.00%> (ø%)