Switch to bare-centos6 single-use nodes

All jobs which previously used centos6 nodes now use bare-centos6
nodes instead. Match the corresponding job names in Zuul's layout to
apply the single-use parameter function to the nodes running them.

Change-Id: I64eae79f1ddbd83b1d404383aa47fc812fbe94fc
Merge "Switch to bare-centos6 single-use nodes"