Switch to gator CLI for testing

[phillebaba]

The Gatekeeper project has released a CLI called gator which enables testing of contraint templates.
https://open-policy-agent.github.io/gatekeeper/website/docs/gator

The tests are simpler than the existing rego tests as they work by passing manifests that are either allowed or disallowed. Currently the gator cli does not support testing of mutations, but neither does our rego tests. I have pushed the branch feature/gator-test-suite which contains an example of how this can be implemented.

Just wanted to check how everyone else feel about changing.

[simongottschlag]

+1 for using the same as them