Automatic merge from submit-queue

e2e-runner: Drop Trusty support in favor of GCI

Now that GCI (Google Container-VM Image) is out, we will start running e2e tests
with it instead of Ubuntu Trusty. This change updates `e2e-runner.sh` to replace
the Trusty related logic with GCI.

_Note that this change has to go in the same time as https://github.com/kubernetes/test-infra/pull/54_

@spxtr Can you review?

cc/ @andyzheng0831 @kubernetes/goog-image