Enable the bot to build for specific CVMFS repo

[trz42]

Currently, the bot's build target CVMFS repository is the default pilot.eessi-hpc.org (as configured in the build container) or any other repository that is configured via the pull request of the software layer. While this works well, it is not so straightforward to build one PR for another repository (e.g., for test purposes). Instead of configuring the target repository via a pull request, the bot's configuration could define target repositories and all necessary customisations. Then, the bot could work as follow to set up a build job:

1. Obtain PR content.
2. Apply changes defined in bot's configuration (the bot could have a default target which does not apply any changes to the PR content ... this would allow that also on the side of the PR one could still built for any repository).
3. Submit job.

[boegel]

Ideally this also covers building and deploying software for something that is not a CVMFS repository at all, could be just an NFS shared filesystem.

I think the bot should be refactored so that is isn't actually aware for what filesystem/repo it is building software.

It should:

• use a script named bot-build.sh to run the build phase
• use a script named bot-test.sh to run the test phase
• use a script named bot-deploy.sh to run the deploy phase

That way, everything that is specific would be in those scripts, and the bot is only aware of the different phases (build-test-deploy), not of how those are run exactly.

The job script in the bot repository, most of the logic should actually be in the bot-build.sh script that comes from the EESSI/software-layer GitHub repo (in the context of EESSI).

edit: separate issue for this: #126

[boegel]

@trz42 This is done, no?