Changes

testenv: remove dead code for [testsuite] prepare=

Remove some WIP code that I intended to use for enabling the mongodb
repository before installing binary packages, to be able to dynamically
install mongodb from there. I solved it differently by just always
having mongodb in the podman image.

This was a dead code path because configs with prepare= in [testsuite]
are currently not valid, and therefore testenv refuses to use these
configs (see keys_valid_testsuite in testenv_cfg.py's verify()).

I have a different use case for running prepare= before running the
testsuite, to replace a module parameter in the testsuite's config. This
will be done in the next patch.

Change-Id: I9bed48612fad690f86017c298372dc6cea22b94b

testenv: add proper [testsuite] prepare= support

Support using prepare= in the [testsuite] section to e.g. change module
parameters:

  [testsuite]
  prepare=sed -i 's/mp_my_parameter := false/mp_my_parameter := true/' Testsuite.cfg

Move the execution time of prepare and clean scripts in
testdir.prepare() after the testsuite config has been copied to the
testdir, so it can be modified by the prepare script.

Change-Id: Ia78a4eba061e5e070f156f75eae1dfa4cede1ee3