crypto: move check for TLS creds 'dir' property

The check for the 'dir' property is being repeated for every
credential file to be loaded, but this results in incorrect
logic for optional credentials. The 'dir' property is mandatory
for PSK and x509 creds, even if some individual files are
optional. Address this by separating the check for the 'dir'
property.

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