Allowed nested resource metadata in POST'd samples

Fixes bug 1302664

Previously, posting samples with nested metadata caused the mongo
driver to fail on the embedded period in a metadata key.

Now, we explicitly unwind the flattened resource metadata before
publishing the sample.

Change-Id: Ibb0980afc880218962328a9b7fe792015d58d1d2
(cherry picked from commit 2e8fa7c48fe4cca5674f895bd3985dc9243a9702)