This adds the logic to increment the revision numbers
for objects whenever there are changes and it exposes
the revision number via a field in the API.
This is handled with a new default service plugin that
subscribes to DB events and bumps revision numbers for
any objects that were modified.
It also handles the logic for bumping the revision number
of a parent in a relationship where the children aren't
top-level neutron objects that would be tracked individually.
This is accomplished with a 'revises_on_change' attribute
on the child models that the service plugin will use to
find the parent and bump its revision.
API tests are included to test the revision numbers
added to each standard attribute enabled object.
Partially-Implements: bp/push-notifications
Change-Id: I476d3e03c8ee763cc4be6d679fe9f501eb3a19b5
4e8cc68349