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 Job ID Name Coverage
  Build
passed #10145
docker
autoconf

00:00:54

passed #10146
docker
cmake

00:00:25