Add basic Gitlab CI configuration

There are certain issues with the Buildbot. Most probably, Ralf can fix them once he is back. However, we should move from the Buildbot to GitLab CI anyway. So, I even disabled the Buildbot webhook.

Daint will be decommissioned pretty soon. We run tests for PGI and Cray compilers there. I think we can simply drop PGI tests because the compiler is very old anyway. As for Cray, we will have to find another solution in the future.

We do not have GitLab runners on Breeze yet. We will enable the respective tests as soon as we get one.

This MR implements a simple one-to-one transfer of the Levante tests from the Buildbot to GitLab CI. The implementation will be made more idiomatic in a follow-up MR but it is good enough for the time being.