Add db.dnsdomain_get_all() method

Add a dnsdomain_get_all() method to the db API.  This method returns the
full model for each dnsdomain in the database.  There is a method
similar to this, dnsdomain_list(), but it doesn't return all of the
data, just one column from each of the rows.  That doesn't really map
well to objects, so make a new method that will be used by the
DNSDomainList object.

Part of blueprint nova-network-objects

Change-Id: I8753cf1e641a1a6040a32a9a43e8b29f2d32182f
Merge "Add db.dnsdomain_get_all() method"