[Enhancement] add PROD variable to RHEL makefiles

- Added $(PROD) to RHEL 6 & 7 Makefiles to match Fedora Makefile as well as make it easier to template
[Enhancement] add PROD variable to Openstack and RHEVM3 Makefiles
Merge pull request #284 from redhatrises/makefile_updates

Makefile updates - add $(PROD) variable