Make _get_compute_info() use objects

This removes a direct call to the conductor api for getting service
info and replaces it with an object call. We still need to convert
the compute nodes to primitives for the live migration compute info
blob, but once that is converted to objects, we should be good.

Related to blueprint compute-manager-objects-juno

Change-Id: Ic8213bac5ce4754bfc95d7b47e8b9d4a65dffcc6
Merge "Make _get_compute_info() use objects"