Cells: Add support for compute HostAPI()

This will allow the hosts API extension to work. Calls to get service
information from the DB are proxied via cells.

When using the host extension, the 'id' and 'host' fields for
service/compute will be modified to have the cell prepended to it.  One
can do a 'show' on a 'path!to!cell@id' to see details for a particular
host in the 'path!to!cell' cell, for instance.

Implements bp/nova-compute-cells

Change-Id: I8137fa64d2578acfc3d2457c231de2b0d69b96d8
Merge "Cells: Add support for compute HostAPI()"