gitlab/gitlab-ci.yml: Include salsa-ci pipeline instead of our own.

4 jobs for master in 7 minutes and 22 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed #310642
buster
buster

00:04:39

passed #310644
centos7
centos7

00:04:20

passed #310641
sid

00:07:11

passed #310643
stretch
stretch

00:04:56