Enabled the EPEL repo on all of the nodes in the Vagrantfile.

[tima]

A number of the exercises uses packages in the EPEL repo. As a convenience, all of the nodes should have that repository enabled while bootstrapping to eliminate one more manual step and any confusion that could arise from missing that step.

[aoyawale]

do we want to add ansible in the vagrant file like how https://github.com/dfederlein/ansible-tower-demo/blob/master/Vagrantfile has it?