Ensure every service plan has a unique_id

Originally, service plans were identified by their provider-label-name
tuple. Service brokers (formerly known as gateways) would use those values to
determine the desired plan when provisioning.

We introduced unique_id as a better way to uniquely identify plans (and
services). Our internal brokers use this unique_id attribute (though
third party brokers may not yet).

We want to encourage its use by requiring that Cloud Controller send it
with all provision requests, but we can't yet require that the broker
send it with catalog update requests. Until then, we'll generate a
default value.

[#55995896]