Cells: Add cells API extension
Cells: Add cells API extension

Enables cells-related functionality such as adding neighbor cells,
listing neighbor cells, and showing the capabilities of the local cell.

Implements blueprint nova-compute-cells

DocImpact

Change-Id: Iacc1e68721a7867aeaf233903cbe6d18f09ad96e

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