SIGMON-124: CI for notebooks

WARNING Merge the lhc-sm-hwc first so we don't have notebooks writing to eos!

Context

• ticket: https://its.cern.ch/jira/browse/SIGMON-124
• The goal of the task was to introduce a CI for notebooks so all of them got executed on merge to dev or master
• lhc-sm-hwc merge request: lhc-sm-hwc!25 (merged)

What was done

• new stage/job introduced in gitlab-ci which executes the notebooks on commit to dev or master
• the logic of obtaining the version of lhc-sm-hwc extended - env used as a fallback when the provided __init__ file is not accessible

TODO

• There are some follow-up tasks (https://its.cern.ch/jira/browse/SIGMON-141, https://its.cern.ch/jira/browse/SIGMON-142 and https://its.cern.ch/jira/browse/SIGMON-143)

QA

• template link to sonar issues: http://sonar.cern.ch/project/issues?branch=SIGMON-124_notebooks_CI&id=lhc-sm-api&resolved=false
• code was unit tested
• acceptance tests were not written, because it is mostly a configuration

Review objectives

• assessment of code functionality
• assessment of code quality
• identifying potential refactorings / logic common to other projects
• knowledge sharing