Fix minor inconsistency in models code
Fix minor inconsistency in models code

We have an IdMixin. All the other objects with an ID here use it.
No point in inconsistently defining 'id' with the Amphora model.

Change-Id: I7758c08536e13d3ef653a604bcc74e1896c7d143

openstack/octavia,openstack/octavia,openstack/octavia