Attributes which are mandatory (ie, required for object creation) should
not be removable. However, some attributes (such as Node.chassis_uuid)
are not required for object creation, but should not be removable if
they are set. This commit does the following:
- rename JsonPatchType.mandatory_attrs to non_removable_attrs to better
describe its meaning,
- change its return type to set-of-strings for faster lookup
- ensure all mandatory attributes on the type being patched are
included in the set of non-removable attributes,
- add a new field, JsonPatchType._extra_non_removable_attrs, which
should be a set of attributes that are not required for creation but
should not be removed if set.
Since the object to be patched does not exist at patch-validation time,
we leave the validation logic in methods of JsonPatchType and
subclasses. This means introspecting the types to be patched.
Closes-Bug: #1284781
Change-Id: I2bb7fed2c776c8d63535c5ee19cdc218e57806e3
34b3589ea3