Add continuous gitlab-qa to continuous load on staging

This will help us to get a more realistic traffic pattern. To do this we will want to get results into prometheus via the pushgateway gitlab-org/gitlab-qa#345 (closed).

We can start with a very small subset of test that should always pass:

Something like:

GITLAB_USERNAME=gitlab-qa GITLAB_PASSWORD=*** gitlab-qa Test::Instance::Staging -- --tag smoke

done at intervals. See https://gitlab.com/gitlab-org/quality/staging for examples.