User had a system that refused to store keys into the kernel keyring.
Any operation at all on the keyring would return "Key has been revoked".
Wrap the operations in a try/except so we can ignore keyring failures.
This also adds per-principal sessions. The principal name is stored
in the session key so switching principals in the ccache doesn't
require clearing the keyring.
https://fedorahosted.org/freeipa/ticket/2880