Big Switch: Check for 'id' in port before lookup

Check for the presence of the 'id' key before trying
to do a lookup based on the value. This is necessary
because a 'fields' param to get_port(s) may exclude
the 'id' field before the host_id lookup.

Conflicts:
	neutron/tests/unit/bigswitch/test_restproxy_plugin.py

Closes-Bug: #1358668
Change-Id: If30567d5deaeabd2de6c6287fac7a81695a41cc5
(cherry picked from commit 295efa00081c770a57a5541652d9d3ffd5c1074d)
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.

Conflicts:
	neutron/common/constants.py

Closes-Bug: #1357379
Related-Bug: #1338880
Change-Id: I6537bb1da5ef0d6899bc71e4e949f2c760c103c2
(cherry picked from commit 74d10939903984d5f06c1749a8707fa3257e44ff)
Merge "Big Switch: Check for 'id' in port before lookup" into stable/icehouse
Merge "Forbid regular users to reset admin-only attrs to default values" into stable/icehouse