Do not auto-trigger gitlab CI pipeline on PRs

For security and resources utilization reasons, do not auto-start CI
for opened/updated PRs.

A member of the kubernetes-incubator github org has first to approve
that the PR is reasonable to test by putting the "ci check this" into
the PR's comments.

If approved that way, the CI pipeline starts as always. Only the 1st step
of the pipeline is premoderatied, the rest will follow each over on
success.

Signed-off-by: Bogdan Dobrelya <bogdando@mail.ru>