Support YAML alias/anchor usage in .gitlab-ci.yml

This allows to reuse one job as a template for another one:

```
job1: &JOBTMPL
  script: execute-script-for-job

job2: *JOBTMPL

```

This also helps to solve some of the issues in #342

Signed-off-by: Pascal Bach <pascal.bach@siemens.com>
Signed-off-by: Fabio Huser <fabio.huser@siemens.com>