Allow updating services when quota is almost reached

The MaxServiceInstancesPolicy assumes that all requests are to create a
service instance, and therefore checks whether the current quota is less than
the allowed quota, rather than <=. That's fine, but the same validation is used for update.
This is a problem when you're exactly at the quota limit (e.g. used services=20,
allowed services=20) and try to, for example, rename a service instance. You 
get a message saying you've exceeded your quota, which you haven't.