The image conversion plugin does a processutils exec(), which needs
to spawn python for prlimit support. Under uwsgi, sys.executable
points to uwsgi itself, which won't work in this case. This introduces
a [wsgi]/python_interpreter config option (because I don't think
there is any way to get this from uwsgi itself) which we use for
the exec. By default, it's sys.executable, which is what is used
right now so nobody should notice a change unless they need it.
Note: Making this depend on the devstack change to remove the wsgi
import restriction so we can get a test on it.