CentOS 7 installation fails without EPEL

[joerocklin]

As reported in rabbitmq/chef-cookbook#189 - the erlang package fails to install due to changes in dependent package availability. Adding the yum-epel recipe resolves the issue.

[cmluciano]

This seems to be similar to rhel 5. Fix incoming

[cmluciano]

This will probably also fix rabbitmq on centos 7