Fix typo in compute node mac address definition

The compute node definitions in the sample cobbler.yaml file
provided by puppet_openstack_builder have a typo: each has
a "mac-addres:' entry instead of a 'mac-address:' entry.
This patch corrects the typo.  The build/control/swift/load
balancer node defintions don't have this issue.

Closes-Bug: #1269656