Forbid regular users to reset admin-only attrs to default values

Forbid regular users to reset admin-only attrs to default values

A regular user can reset an admin-only attribute to its default
value due to the fact that a corresponding policy rule is
enforced only in the case when an attribute is present in the
target AND has a non-default value.

Added a new attribute "attributes_to_update" which contains a list
of all to-be updated attributes to the body of the target that is
passed to policy.enforce.

Changed a check for whether an attribute is explicitly set.
Now, in the case of update, the function should not pay attention
to a default value of an attribute, but check whether it was
explicitly marked as being updated.

Added unit-tests.

Closes-Bug: #1357379
Related-Bug: #1338880
Change-Id: I6537bb1da5ef0d6899bc71e4e949f2c760c103c2