parallel execution

The validate-all jobs are approaching 45 minutes in runtime, which is uncomfortably close to the limit of one hour that is imposed per job.

We should filter the available repositories so that we only actually test a repository if this test returns true:

($ENV{CI_NODE_TOTAL} == 1) || ($reponumber % $ENV{CI_NODE_TOTAL} + 1) == $ENV{CI_NODE_INDEX}

... where $reponumber is an index that is incremented on every repository which we want to check. This way, we can then use GitLab's parallel feature to increase the number of jobs and avoid the timeout.