certdb: use custom object for trust flags
Replace trust flag strings with `TrustFlags` objects. The `TrustFlags`
class encapsulates `certstore` key policy and has an additional flag
indicating the presence of a private key.
https://pagure.io/freeipa/issue/6831
Reviewed-By: Stanislav Laznicka <slaznick@redhat.com>
Reviewed-By: Martin Babinsky <mbabinsk@redhat.com>