Add ZOOKEEPER_HOSTS as an optional property

[style95]

Description

As ZOOKEEPER_HOSTS is removed from the required properties, it is no longer transformed into the whisk config from the environment variables. But when the instance id assigned is used, the zookeeper host becomes a required property. This change is to make it an optional property.

Related issue and scope

• [x] I opened an issue to propose and discuss this change (#5399 )

My changes affect the following components

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

Types of changes

• [x] 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:

• [x] I signed an Apache CLA.
• [x] 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 #5435 (731eb8b) into master (951ce58) will increase coverage by 18.08%. The diff coverage is 50.00%.

:exclamation: Current head 731eb8b differs from pull request most recent head a066119. Consider uploading reports for the commit a066119 to get more accurate results

@@             Coverage Diff             @@
##           master    #5435       +/-   ##
===========================================
+ Coverage   58.52%   76.60%   +18.08%     
===========================================
  Files         241      241               
  Lines       14632    14634        +2     
  Branches      616      607        -9     
===========================================
+ Hits         8563    11211     +2648     
+ Misses       6069     3423     -2646     

Files Changed	Coverage Δ
...la/org/apache/openwhisk/core/invoker/Invoker.scala	71.60% <50.00%> (+1.98%)	:arrow_up:

... and 135 files with indirect coverage changes

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[style95]

Without this change, invokers with the ID assignor can not be deployed.

[ningyougang]

LGTM 👍