Add GCE pre-submit that always runs for master

[hakman]

/kind feature

1. Describe IN DETAIL the feature/behavior/change you would like to see.

We already have a number of periodic tests for GCE and would be nice to not break it with PRs that seem unrelated, or even forget to run these tests for GCE related PRs.

2. Feel free to provide a design supporting your feature request.

Already have pull-kops-e2e-k8s-gce as a template, we just need to make it run automatically run for all/selected PRs from master.