Move nova/savanna to only use bare-precise nodes

The earlier changes were faulty as it's not actually possible to
select precise||bare-precise nodes because zuul needs to send
Jenkins the offline-on-complete flag with the job request (and
can not know which node will run it).

Change-Id: Ice0adbaa5629ee289f39f7bcddee7e01bfb2b4c3
Merge "Move nova/savanna to only use bare-precise nodes"