[gatekeeper] add various e2es

[sozercan]

add tests to check violations for:

• [ ] update
• [ ] delete (requires k8s v1.15)
• [ ] selectors (like https://github.com/open-policy-agent/gatekeeper/blob/master/example/constraints/all_pod_must_have_gatekeeper_namespaceselector.yaml)
• [ ] bad constraints/templates
• [ ] dry run

[sozercan]

also, add matrix of tests using different versions of kind and go