We're close to the limit on some testing nodes, so, while building
a CentOS image:
1) Delete files as soon as we no longer need them.
2) Do not copy kernel/ramdisk unnecessary
Move the libvirt pool for testing VMs to /opt since some testing
nodes have small root partitions.
Also move irrelevant vars out of zuul definitions.
Change-Id: I2a34b19ee823109899dc21031e663a3ea848a5c5
777ca793c0