Silence Paramiko deprecation warnings

This works around a problem in Paramiko, but which looks like it's a
Ganeti problem:

[cluster] root@node4:~# gnt-node add --readd node3
/usr/lib/python2.6/dist-packages/Crypto/Util/randpool.py:40:
RandomPool_DeprecationWarning: This application uses RandomPool, which
is BROKEN in older releases.  See http://www.pycrypto.org/randpool-broken


  RandomPool_DeprecationWarning)

To silence this, we do an ugly thing (since Python 2.4 doesn't support
context managers): manually reset the warnings filter, which resets
any -W command line flags (hopefully noone calls this tool manually so
often).

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>