Default to ctx user/project ID in sample POST API

Fixes bug #1202143

Avoid RPC failure when project and user IDs not explicitly
specified in POST'd sample.

Instead default to identity in current context.

Change-Id: Id1368c7ccf730bc62bc2b32247266e87482844cb
(cherry picked from commit 1d0b6397d468d8527ad3b388ee2f2b1714f3c38d)