Rename zones table to cells and Instance.zone_name to cell_name
Rename zones table to cells and Instance.zone_name to cell_name

Vish made an executive decision to use the term 'Cell' to refer to the
former 'Zone'.  :)  This fixes up the core code required for the future
Cells branch.

Change-Id: Iff0fb01f0c2862a646bfb2d3fd4e218f1d2b7a93

citrix-openstack-build/oslo.versionedobjects,openstack/oslo.versionedobjects