Make installation of stack steps conditional

Description

Some demos have requirements that can only be installed at stack installation time. Examples are: airflow dags, jupyter notebooks. These cannot be installed by the demo it's self because that's too late.

Currently some stacks install resources that only make sense as part of demos. This can be confusing and leads to stack pollution with unnecessary steps and resources.

Proposed solution

Introduce some mechanism that allows stacks to create resources conditionally.

In the examples above, stacks should install airflow dags and jupyter notebooks conditionally. The corresponding demos would enable the condition while the stacks would not.

Alternatively, demo stacks could be separated from the "official" stacks.

Beginne damit nachzuverfolgen, wie die Stack-Installation und die Demo-Auswahl in stackablectl behandelt werden, insbesondere die Pfade, die Airflow DAGs und Jupyter notebooks installieren. Definiere einen Mechanismus, mit dem Demos bedingte Ressourcen aktivieren können, während offizielle Stacks diese auslassen, oder ermittle, wie separate Demo-Stacks funktionieren würden; erledigt ist dies, wenn unnötige Demo-Ressourcen standardmäßig nicht mehr installiert werden.