add support for etcd client authentication

[bdoyle0182]

Description

Allows connecting to an etcd cluster with authentication setup. Can just define etcd username and password through config like other db provider configs.

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 #5269 (9d9c779) into master (8843579) will decrease coverage by 65.95%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master   #5269       +/-   ##
==========================================
- Coverage   70.54%   4.59%   -65.96%     
==========================================
  Files         238     238               
  Lines       14080   14084        +4     
  Branches      576     588       +12     
==========================================
- Hits         9933     647     -9286     
- Misses       4147   13437     +9290     

Impacted Files	Coverage Δ
...la/org/apache/openwhisk/core/etcd/EtcdClient.scala	0.00% <0.00%> (-6.56%)	⬇️
...ala/org/apache/openwhisk/core/etcd/EtcdUtils.scala	0.00% <ø> (-30.62%)	⬇️
.../openwhisk/core/loadBalancer/FPCPoolBalancer.scala	0.00% <0.00%> (ø)
...he/openwhisk/core/invoker/FPCInvokerReactive.scala	0.00% <0.00%> (ø)
...rg/apache/openwhisk/core/scheduler/Scheduler.scala	0.00% <0.00%> (-0.68%)	⬇️
...ain/scala/org/apache/openwhisk/spi/SpiLoader.scala	0.00% <0.00%> (-100.00%)	⬇️
...n/scala/org/apache/openwhisk/utils/JsHelpers.scala	0.00% <0.00%> (-100.00%)	⬇️
...scala/org/apache/openwhisk/common/Prometheus.scala	0.00% <0.00%> (-100.00%)	⬇️
...scala/org/apache/openwhisk/core/FeatureFlags.scala	0.00% <0.00%> (-100.00%)	⬇️
...ala/org/apache/openwhisk/common/ConfigMXBean.scala	0.00% <0.00%> (-100.00%)	⬇️
... and 197 more

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[style95]

LGTM