Use kind to run integration tests on TravisCI

[dgageot]

This is a first try at using kind for our integration tests.

• It creates a kind cluster in Travis.
• It runs the test suite in the same container that's currently used on kokoro.
• I had to add a few lines of code in skaffold to handle kind clusters. The main change is that docker images have to be explicitly loaded before they are used by a k8s workload.

[codecov-io]

Codecov Report

Merging #2196 into master will decrease coverage by 0.02%.
The diff coverage is 44.44%.

@@            Coverage Diff             @@
##           master    #2196      +/-   ##
==========================================
- Coverage   58.55%   58.52%   -0.03%     
==========================================
  Files         188      188              
  Lines        7882     7890       +8     
==========================================
+ Hits         4615     4618       +3     
- Misses       2877     2881       +4     
- Partials      390      391       +1

Impacted Files	Coverage Δ
pkg/skaffold/runner/build.go	54.16% <0%> (-14.26%)	⬇️
cmd/skaffold/app/cmd/config/util.go	59.16% <100%> (+1.04%)	⬆️

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[dgageot]

@tejal29 @balopat @nkubala This PR is ready to be reviewed. Be aware that it doesn't run all the tests we used to run on kokoro. Because it has no gcp credentials, it can't use cloud build or have kaniko push to gcr. Tell me what you think

[balopat]

LGTM, let's start using this!