Handle the slave bare flag properly in nodepool.

Just to be confusing our bare-precise images are not actually bare
jenkins slave images and things will break if they are. Make the
jenkins::slave bare flag selectable in the single_use_slave manifest and
set it to true on the bare precise nodes (keep it set to false on
anything running devstack or similar).

Change-Id: I88d0a7f7b9c188ac3547b9ffab57c5fbce4f7a10
Merge "Handle the slave bare flag properly in nodepool."