CI: compute coverage in seperate CI job

GitLab by default averages the code coverage values from the different jobs. This is in most cases not correct, since jobs could target different parts of the code. This MR correctly computes the code coverage in a separate job.

It also limits the execution of Jobs to Merge requests and the default branch, and deploy tags. Introduced with https://gitlab.com/gitlab-org/gitlab-ce/issues/15310