Use focal release for travis ci

[jiangpengcheng]

Seems travis ci is broken again, trying to fix it

Description

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
• 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-io]

Codecov Report

Merging #5052 (e8950b5) into master (a6ad9e4) will decrease coverage by 11.39%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           master    #5052       +/-   ##
===========================================
- Coverage   82.65%   71.25%   -11.40%     
===========================================
  Files         206      199        -7     
  Lines       10000     9735      -265     
  Branches      438      435        -3     
===========================================
- Hits         8265     6937     -1328     
- Misses       1735     2798     +1063     

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	3.33% <0.00%> (-86.67%)	⬇️
...la/org/apache/openwhisk/http/BasicRasService.scala	16.66% <0.00%> (-83.34%)	⬇️
...enwhisk/connector/kafka/KamonMetricsReporter.scala	0.00% <0.00%> (-83.34%)	⬇️
... and 49 more

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[style95]

LGTM

[ningyougang]

In my env, seems here has relation with python version, test as below:

• If use python3 as default: ln -s /usr/bin/python3 /usr/bin/python

  lines(0) is ['nodejs:10', 'blackbox']

• If use python2 as default: ln -s /usr/bin/python2 /usr/bin/python

  lines(0) is [u'nodejs:10', u'blackbox']

So here meets 2 cases.