pybind: fix Rados.conf_parse_env test

This happens after we connect, which means we get ENOSYS always.
Instead, parse_env inside the normal setup method, which had the added
benefit of being able to debug these tests.

Backport: dumpling
Signed-off-by: Sage Weil <sage@inktank.com>
(cherry picked from commit 6ef1970340c57d6e02f947348fb38882b51d131c)