[Scheduler Enhancement] Add test cases to make sure an invoker properly boots up in terms of ETCD keys.

[style95]

Description

These test cases would make sure invokers properly remove ETCD keys for health actions after becoming healthy.

Related issue and scope

• I opened an issue to propose and discuss this change (#????)

My changes affect the following components

• API
• Controller
• Message Bus (e.g., Kafka)
• Loadbalancer
• Scheduler
• Invoker
• Intrinsic actions (e.g., sequences, conductors)
• Data stores (e.g., CouchDB)
• Tests
• Deployment
• CLI
• General tooling
• Documentation

Types of changes

• Bug fix (generally a non-breaking change which closes an issue).
• Enhancement or new feature (adds new functionality).
• Breaking change (a bug fix or enhancement which changes existing behavior).

Checklist:

• I signed an Apache CLA.
• I reviewed the style guides and followed the recommendations (Travis CI will check :).
• I added tests to cover my changes.
• My changes require further changes to the documentation.
• I updated the documentation where necessary.

[codecov-commenter]

Codecov Report

Merging #5271 (86df066) into master (639c4a9) will decrease coverage by 7.22%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #5271      +/-   ##
==========================================
- Coverage   70.36%   63.13%   -7.23%     
==========================================
  Files         238      238              
  Lines       14084    14084              
  Branches      588      588              
==========================================
- Hits         9910     8892    -1018     
- Misses       4174     5192    +1018     

Impacted Files	Coverage Δ
...che/openwhisk/core/invoker/LogStoreCollector.scala	0.00% <0.00%> (-100.00%)	⬇️
...core/database/cosmosdb/RxObservableImplicits.scala	0.00% <0.00%> (-100.00%)	⬇️
...ore/database/cosmosdb/cache/CacheInvalidator.scala	0.00% <0.00%> (-100.00%)	⬇️
...e/database/cosmosdb/cache/ChangeFeedConsumer.scala	0.00% <0.00%> (-100.00%)	⬇️
...core/database/cosmosdb/CosmosDBArtifactStore.scala	0.00% <0.00%> (-95.85%)	⬇️
...sk/core/database/cosmosdb/CosmosDBViewMapper.scala	0.00% <0.00%> (-93.90%)	⬇️
...tabase/cosmosdb/cache/CacheInvalidatorConfig.scala	0.00% <0.00%> (-92.31%)	⬇️
...a/org/apache/openwhisk/http/BasicHttpService.scala	3.17% <0.00%> (-87.31%)	⬇️
...enwhisk/connector/kafka/KamonMetricsReporter.scala	0.00% <0.00%> (-83.34%)	⬇️
...e/database/cosmosdb/cache/KafkaEventProducer.scala	0.00% <0.00%> (-78.58%)	⬇️
... and 39 more

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