Check if "kerberos" package can be imported at runtime
(in __init__.py). If not, everything works as before.
If "kerberos" package can be imported register a new
urllib handler (defined in urllib_kerb.py) which adds
support for kerberos auth.
This urllib handler (urllib_kerb.py) is only triggered
on 401 (Unauthorized) response, and we try to auth
with kerberos. If unsuccessful, next 401 handler (if any)
is triggered.
Change-Id: I1a47e455aa14535a124df950994718a11d7e4f57
4152a0138b