Options to enable/disable features

[MakisH]

Currently, the vm contains everything we support by default, but this ends up being a very large image that take a lot of time to build.

Having options to enable/disable features would be useful, e.g., in the following cases:

• Debugging a specific feature of the vm / ecosystem.
• Needing a reference system just for preCICE itself
• Needing a vm with specific versions of some components, for reproducibility.

This is easy, I will work on it sometime in the next few weeks.

[MakisH]

We could also build a barebones VM and then provide the provisioning scripts to the user to install whatever they need on demand. This would be helpful for development or the VM, as the VM would build faster.