shade defaults to returning everything under the sun in every form
possible in order to ensure maximum backwards compatability - even with
systems that are not shade itself. However, passthrough fields from
somewhere else could change at any time. This patch adds an opt-in flag
that skips returning passthrough fields anywhere other than the
properties dict.
Change-Id: I7071a406965ed373e77f9592eb76975400cb426b
fa80a51d0f