This is needed to build docs without installing BuildStream.

  o Added feature to allow running shell commands

  o Removed unused codepaths to launch bst commands through the script,
    this program is anyway only used to parse and run the session files.

  o Fixed previous patch to run BuildStream uninstalled with
    the subprocess module, as running it in the same interpretor
    causes a bunch of problems.

Since we added support for shell commands in bst2html, this can now be fixed.

This is a part of issue #553

Build documentation without installing

See merge request !605

These aren't official yet, but they should work.