Use the default key for gitab-ci config.

Gitlab now supports a `default` section on the build description,
which sets defaults which can be overridden by specific jobs.

This encapsulates things a little better and avoid duplicating
runner tags.
2 jobs for master in 54 seconds (queued for 3 seconds)
Status Name Job ID Coverage
  Build
passed autoconf #10145
docker

00:00:54

passed cmake #10146
docker

00:00:25