API now passes objects to central

Now that API and Central both use objects, the communication between
the two uses objects rather than dicts.  The create (POST) operations
are the ones that are affected by this change.

Change-Id: Ife31ae7e71632dc327e6d7843cad1fdbaea7337d
Implements: blueprint mdns-designate-central-api-comm
Merge "API now passes objects to central"