Introduce a JVMManager to handle JVM startup details

Introduces a separation in Manager so that we can distinguish "JVM starting" concepts from "Package management" concepts.

The ultimate goal is to allow users to configure the JVM before it starts (and for libraries to confirm that the JVM conforms to their requirements). This is coming from pytimber, which needs to be able to define a larger max heap size. A follow up MR will likely set a bigger default than the one that OpenJDK offers by default. If that doesn't suffice (or people need to tweak it) they will have a public API to change the value:

import cmmnbuild_dep_manager
cmmnbuild_dep_manager.JVM.startup_arguments.max_heap_size = '16g'