crypto: reduce duplication in handling TLS priority strings

crypto: reduce duplication in handling TLS priority strings

The logic for setting the TLS priority string on a session object has a
significant amount of logic duplication across the different credential
types. By recording the extra priority string suffix against the
credential class, we can introduce a common method for building the
priority string. The TLS session can now set the priority string without
caring about the credential type.

Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>