Fix compute_api object handling code in cells messaging

This fixes the calling convention for _from_db_object() in the
object handling code for calling compute_api in cells. It also
changes the guinea pig method from reboot to backup so that
we can convert reboot to objects in a few patches.

Related to blueprint compute-api-objects

Change-Id: I9d359cb13fc054375c2a7c1e45e6770f274c8449