UcsHandle instance has no attribute 'cookie', fix

This fixes the problem reported on the mailing list:
http://www.freelists.org/post/racktables-users/Getting-internal-server-error-while-trying-to-authenticate-Cisco-UCS,4

Quoting Jeff Silberman:
"There is no ".cookie" attribute. There's a "._cookie", but it's an
internal attribute. There are no public access methods for session
cookies, likely for security reasons. So it shouldn't have been there in
the first place."