add 'triggers' keyword to gitlab-ci.yml 'only' and 'except' fields to allow control over whether triggers will cause jobs to run

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

- Offloads uploading to GitLab Workhorse
- Use /authorize request for fast uploading
- Added backup recipes for artifacts
- Support download acceleration using X-Sendfile