[feat] Pass parent CI image name and ReFrame verbosity level in the generated Gitlab child pipelines

[mahendrapaipuri]

• This PR emits docker image name, if defined in the parent CI script, in the generated CI scripts.
• If the user chose to increase the verbosity in the parent CI script, the child scripts will also inherit the same level of verbosity. In the current implementation, there is no verbosity and it is hard for the user to debug the test failures.
• Tested on simple CI pipeline.

Fixes #2134

[jenkins-cscs]

Can I test this patch?

[teojgo]

Ok to test

[codecov-commenter]

Codecov Report

Merging #2137 (8f5f123) into master (5dffa31) will decrease coverage by 0.00%.
The diff coverage is 80.00%.

@@            Coverage Diff             @@
##           master    #2137      +/-   ##
==========================================
- Coverage   85.97%   85.96%   -0.01%     
==========================================
  Files          54       54              
  Lines        9644     9649       +5     
==========================================
+ Hits         8291     8295       +4     
- Misses       1353     1354       +1     

Impacted Files	Coverage Δ
reframe/frontend/ci.py	91.42% <80.00%> (-1.91%)	⬇️

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[mahendrapaipuri]

Does it sound logical to add a new configuration section for CI pipelines to define parameters like before-script, after-script, expire_in (for artifacts) so that they will be emitted in the generated yaml files?

[vkarak]

@mahendrapaipuri thanks a lot for your PR! I have just a couple of minor comments. Also we should probably document these additions somewhere in the user documentation.