Handle parameter reading and test case execution together

For reasons I don't understand, the XML option reading and actual test
case execution are at different places in the code. This is problematic,
because what this led to is that options are not passed as function
arguments, but indirectly through class members. This led to a lot of
"indirect code"; it's a total mess

To improve this, move them together, so that parameters can be passed
directly as function arguments later on.

Because we reference the globals, mark them inside
ExecuteActionWithParam(). That is a Python antipattern, but necessary at
this moment. I believe that we can remove the following globals over
time:

- SSH (with new cls_cmd, which we don't need to pass)
- EPC (replaced through a class similar to ue_module, i.e., refer to
  core networks on specific hosts through an ID, instead of a global)
- HTML (should use templating, where individual functions give back a
  dictionary of what they want to print, and a final pass assembles the
  entire HTML file)
- RAN/SCA/PHYSIM/CLUSTER/ldpc (can be replaced with stateless scripts
  build/run eNB/gNB on a specific node)