Run integration tests in Travis CI

[samos123]

• Change integration tests to use docker hub instead of GCR due to bug
  in library that requires authentication with gcr.io even for public
  images. See kaniko requires authentication for base images that are in gcr.io #966 for bug tracking this.
• Make uploading to GCS bucket configurable through a flag --uploadToGCS
• Utilize a locally deployed docker registry in travis CI to remove
  dependency of needing to authenticate with GCP. This requires host
  networking such that we can access the registry on localhost:5000
• Use the commit that's being tested for TestGitBuildcontext
• Remove duplicate GitBuildContext case that's now covered by default in
  travis CI.

Related to #947

Submitter Checklist

These are the criteria that every PR should meet, please check them off as you
review them:

• Includes unit tests
• Adds integration tests if needed.

See the contribution guide for more details.

Reviewer Notes

• The code flow looks good.
• Unit tests and or integration tests added.

[cvgw]

ok to change this one from distroless/base to debian?

[samos123]

since there is no reason to use distroless for the test case. Distroless is only available on GCR and there is an issue with GCR when you're not authenticated with gcloud.

[cvgw]

Very nice 👍

[tejal29]

This is great!!
My only concern is we have increased the Travis CI time from 6 mins to 20 mins.
Can we create a Matrix CI job and run make integration be another sub job. That way we can reduce the wait time to ~14 mins ?
What do you think?

Would you be able to do this in this PR ? or want to follow up in another PR?

[samos123]

Let's separate that in another PR. I think 20 mins is still acceptable for a full integration testing run since previously it was only running unit tests.

[samos123]

also I suggest to keep kokoro running for the the next few weeks at least. This way we can easily see whether some issues might not be catched with a local registry vs using gcr.