Add smoke test for GKE

[hidalgopl]

We currently have terraform scripts for running KIP in GKE cluster. Recently, we had to fix it. We should be more proactive; adding smoke test similar to the one we recently added for EKS would be good first step.

Possible smoke test scenario would be provisioning GKE cluster with current default GKE version; deploying KIP, deploying nginx server on regular node, deploying curl-er pod onto KIP and checking connectivity between those two.

Ideally we would like to run EKS and GKE smoke tests in parallel; I'm not sure if it's feasible with current travis CI configuration. We may think about switching to github actions, so docker images are built as first step; then smoke tests are run in parallel.