Introduce travis-based integration testing

[Skeen]

This PR introduces a .travis.yml file, which tests the minikube statefulset example.

In order to enable Travis on this repository (assuming the PR gets merged), please follow: https://docs.travis-ci.com/user/tutorial/

Setup shouldn't take more than 5 minutes.

Issues

Upon creating this minor test-suite, I've hit upon #86 and #85 both of which affect the master branch.

• Add ClusterRole to allow listing pods #86 has a quick workaround:
  kubectl create clusterrolebinding default-admin --clusterrole cluster-admin --servicaccount=default:default
• Mongo error because node found twice #85 seems to require changes to the sidecar itself, but does only seem to affect certain kubernetes versions / bootstrappers (affects kubernetes v1.10.0, but not v1.12.0).

The build log for the #86 effected build can be seen here:

• https://travis-ci.com/Skeen/mongo-k8s-sidecar/builds/93256103

While the sidecar error produced with kubernates v1.10.0 for #85 can be seen here:

• Mongo error because node found twice #85 (comment)

Credits

The .travis.yml file is inspired by: https://github.com/LiliC/travis-minikube and this blog post: https://blog.travis-ci.com/2017-10-26-running-kubernetes-on-travis-ci-with-minikube