RapiClient: fix multi-authentication in Python 2.6

In Python 2.6 the urllib2.HTTPBasicAuthHandler has a "retried" count for
failed authentications. The handler fails after 5 of them. To solve this
we reset the handler's "retried" member variable to 0 after every
successful request. This is a bit ugly, but makes the client work again
for more than 5 requests under all versions of Python.

Note that the digest authentication handler has a reset_retry_count()
method to do this, but the method is not defined for the basic
authentication handler, so we must reset the variable itself.

This member variable is unused in 2.4 and 2.5, so the change doesn't
affect the client under older Python versions.

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>