Use username from node information if available

Nodepool knows the username that you should ssh with at image build time
and includes this information in the node data. Zuul should use this
username for the executor target.

Change-Id: I1e677061b9fd495b192d25a5825362c81e40d0c6
Depends-On: Ife0daa79f319aea04ed32513f99c73c460156941
7 files changed