Fix session cookies
The CLI was not using session cookies for communication with IPA API.
The kernel_keyring code was expecting the keyname to be a string, but
in python 2 a unicode was supplied (the key is built using
ipa_session_cookie:%principal and principal is a unicode).
The patch fixes the assertions, allowing to store and retrieve the cookie.
It also adds a test with unicode key name.
Reviewed-By: Petr Spacek <email@example.com>