Improve commit messagges from the core service

[lorenzo-cavazzi]

Is your improvement request related to a use case or a component design? Please describe.
While using the new config APIs, I've noticed the commit messages created by the core service are a bit cryptic. Specifically, I see:

gunicorn renku.service.entrypoint:app --preload -b 0.0.0.0:8080 --timeout 600 --workers 4 --worke...

For a user, it may be unclear what happened.
It would be nice to have something similar to other APIs, where the message includes the cli equivalent command, like templates.create_project

service: renku init -n "project-config" -s "https://github.com/SwissDataScienceCenter/renku-proje...

Proposal
Having a standard for the commit messages would be great. For the APIs, it could be something like:

• First line: scope (E.G. renku service - update config, renku service - create project)
• Second and following lines: cli equivalent command, or other relevant information to re-run the command where the command is not meaningful (E.G. renku init -n "project-config" -s "https://github.com [...]", renku config set [...]). Do not cut the renku command, or do that only when the number of chars is very high -- currently, it's a bit low

Update
Scope of the issue:

• Align the commit messages coming from the service. Ideally, preserve the CLI equivalent message with a prefix to indicate it's coming from the service. This may not be possible for some commands not having a 1:1 CLI equivalent.
  Info: generally keep the possibility to provide a custom commit_message wherever reasonable, but prevent setting a custom message early (e.g. https://github.com/SwissDataScienceCenter/renku-python/blob/master/renku/service/serializers/datasets.py#L40-L43)
• Evaluate the possibility to extend the commit messages to prevent cutting out important information.
  Info: the length is cut after 100 chars https://github.com/SwissDataScienceCenter/renku-python/blob/master/renku/core/utils/scm.py#L54-L75
  Switching to cut=False to have multi-line messages could be a solution, but we may need another "message body hard limit" variable to prevent some commands from failing due to the excessive length (e.g. renku run or renku dataset add *). Picking a reasonable default length may be hard, being system-dependent.
  P.S. users may be happy to see again the CLI command in the commit: https://renku.discourse.group/t/how-to-see-the-full-command-again/262 . Answer there in case of any significant change.