change default warmed container keeping count to 0 if not configured

[bdoyle0182]

Description

I don't think this feature should be turned on by default. You should be required to configure to keep the compute around rather than it leave one container by default for an extra x minutes (I think the default system config for this is 10min) past the idle timeout. It also isn't very helpful to default to that by namespace because there can be multiple actions per namespace so it's not very helpful to by default leave one container hanging around using resources when the next request may not even be for that action within the namespace.

We can discuss here, but figured I'd just go ahead and open the pr to discuss

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 #5328 (2aada8c) into master (625c5f2) will decrease coverage by 4.86%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master    #5328      +/-   ##
==========================================
- Coverage   81.08%   76.22%   -4.87%     
==========================================
  Files         239      239              
  Lines       14259    14259              
  Branches      575      575              
==========================================
- Hits        11562    10869     -693     
- Misses       2697     3390     +693     

Impacted Files	Coverage Δ
...he/openwhisk/core/invoker/FPCInvokerReactive.scala	0.00% <0.00%> (ø)
...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%)	⬇️
...enwhisk/connector/kafka/KamonMetricsReporter.scala	0.00% <0.00%> (-83.34%)	⬇️
...e/database/cosmosdb/cache/KafkaEventProducer.scala	0.00% <0.00%> (-78.58%)	⬇️
...whisk/core/database/cosmosdb/CosmosDBSupport.scala	0.00% <0.00%> (-74.08%)	⬇️
... and 15 more

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[ningyougang]

LGTM