travis: run deploy as final stage
travis: run deploy as final stage

The `deploy:` section is executed as part of each successful job. That
means the container images and Helm charts are built and pushed many
times during testing after a push/merge event.

By creating a deploy stage and listing it after the tests, it is
guaranteed to run only once after all tests have succeeded. By adding a
condition to only run the stage for push events, the images and Helm
charts should only get built+pushed after a PR has been merged.

See-also: https://docs.travis-ci.com/user/conditional-builds-stages-jobs#Specifying-conditions
Signed-off-by: Niels de Vos <7bda0c0ee7f47171cc808e9e60292cbc5d8140dd@redhat.com>

ceph/ceph-csi,ceph/ceph-csi,ceph/ceph-csi,ceph/ceph-csi