Fixup missing variables for CI.

For CI we had a number of (tightly related) missing variables:
 - The SSH key has to be passed in, not generated in-instance
 - The host is not 192.168.122.1
 - The arch etc all come from the test environment the test is
   running on so we need to read them from the JSON.

Change-Id: Ia373cd2007750a2ce5ad0b1ee18cf3c44591d9dc
Co-Authored-By: Robert Collins <rbtcollins@hp.com>