add exclude nodes setting

[ylwu-amzn]

Description

Add exclude nodes setting.
If user don't want to run ML tasks on some ML nodes, they can use exclude the nodes with this setting.

Issues Resolved

#812

Check List

• New functionality includes testing.
  • All tests pass
• New functionality has been documented.
  • New functionality has javadoc added
• Commits are signed per the DCO using --signoff

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

Codecov Report

Merging #813 (e9d8054) into 2.x (894f983) will increase coverage by 0.28%.
The diff coverage is 78.37%.

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@@             Coverage Diff              @@
##                2.x     #813      +/-   ##
============================================
+ Coverage     84.82%   85.10%   +0.28%     
- Complexity     1598     1622      +24     
============================================
  Files           135      135              
  Lines          5971     6003      +32     
  Branches        570      580      +10     
============================================
+ Hits           5065     5109      +44     
+ Misses          663      648      -15     
- Partials        243      246       +3     

Flag	Coverage Δ
ml-commons	85.10% <78.37%> (+0.28%)	⬆️

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

Impacted Files	Coverage Δ
...rg/opensearch/ml/plugin/MachineLearningPlugin.java	98.82% <ø> (ø)
.../java/org/opensearch/ml/task/MLTaskDispatcher.java	32.94% <0.00%> (ø)
...h/ml/action/unload/TransportUnloadModelAction.java	77.10% <25.00%> (-1.91%)	⬇️
...org/opensearch/ml/cluster/DiscoveryNodeHelper.java	95.23% <83.33%> (+5.84%)	⬆️
...n/java/org/opensearch/ml/model/MLModelManager.java	78.92% <90.90%> (+0.29%)	⬆️
.../org/opensearch/ml/settings/MLCommonsSettings.java	100.00% <100.00%> (ø)
...va/org/opensearch/ml/task/MLPredictTaskRunner.java	82.14% <100.00%> (ø)

... and 4 files with indirect coverage changes

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