Make instance_update() string-convert IP addresses

Since postgres is strongly typed, it gets annoyed if we pass our
netaddr.IPAddress objects to it for an update. This makes the
instance_update() operation string-convert those before passing
them to the sqlalchemy save method.

Related to blueprint compute-api-objects

Change-Id: Ie9d4a59f50e59e80e88a8bb88575b89ce2e59091
Merge "Make instance_update() string-convert IP addresses"