Initial RHEL7/Centos7 support

Install the right EPEL and puppet rpm's for RHEL7/Centos7.

I have tested this on one of the CentOS nightly builds and
run_puppet.sh, install_modules.sh and a puppet apply of
openstack_project::single_use_slave works as expected

Change-Id: I80024d1afdb4e40d5fe9793ab2ec443b887c5fa8
Merge "Initial RHEL7/Centos7 support"