VPNaaS update connection validation is missing important data

[Adit Sarfaty]

In VPNaaS plugin, when updating a connection, the plugin calls driver.validator.validate_ipsec_site_connection with only the updated fields data.
This may not be enough for the validation, since the data of the existing (pre-change) connection is missing.
Any validation that depends on a combination of fields cannot be performed.
Instead, the connection-id or original-connection should be added as argument.