Provide init hook for test quickstarters to better plug into RM

[clemensutschig]

Is your feature request related to a problem? Please describe.
For some quickstarters - like tosca - we want to retrieve external files early in the process, before we build, and only in specific cases - and to commit them to the repository. This is currently not possible.

Describe the solution you'd like
a hook in the init phase of the MRO