enable the bot to execute configurable workflows

[trz42]

Right now the bot executes a relatively fixed workflow (defined by the bot) where the individual blocks are defined by scripts in a target repository.

We should look into making this more flexible to allow for workflows that

• can be defined per target repository
• can be adjusted to new needs without the need to change the bot itself
• allow conditional execution
• etc

Likely this means that the bot needs some kind of workflow engine. In addition to technical considerations such as which engine should be used, there need to be good understanding of what (build, test, deploy, progress, status, etc) can be executed where (frontend, compute node).