Add DNSDomain object

Add a new object, DNSDomain, along with a DNSDomainList object.  This is
needed for converting nova-network to use objects so that we can have it
using conductor.

Part of blueprint nova-network-objects

Change-Id: I2e337ccdf9a23a3536ef6bdcd9bf3e4b825d4d74
Merge "Add DNSDomain object"