Add https://github.com/maistra/maistra-test-tool/ to OpenShift Prow:

This is about only the tide part. maistra-test-tool repo should behave like other maistra repos, meaning if the PR is approved and it has the `okay to merge` label, it should be automatically merged by tide.

A/C:

• activate plugins to maistra-test-tool repo similar to https://github.com/openshift/release/blob/master/core-services/prow/02_config/maistra/istio/_pluginconfig.yaml
• activate tide on this repo - might be dependent on OSSM-3266.