[Request] Add support for alternative environments apart from LCG stacks

[tvoigtlaender]

Is your feature request related to a problem? Please describe.
LCG stacks are a common way to deploy HEP relevant software to a machine, but in some cases they do not provide sufficient flexibility. Some newer features take months to reach the stable LCG stacks. Relying on the LCG stacks also limits on which machines CROWN can run as it needs to have the cvmfs available.

Describe the solution you'd like
An alternative to the use of LCG stacks should be allowed. Options include custom setup scripts or a package manager, but also to compile/run the CROWN executables in a container. The workflow orchestration of CROWN (KingMaker) already utilizes containers when running the CROWN compiled code on the batch system. A related request has been opened for KingMaker.