yes EPEL is required, but packstack should be auto enabling EPEL where required (consider multi host installs also). What might be happening is that the EPEL release RPM was already installed but you had it explicitly disabled in /etc/yum.repos.d/epel.repo for some reason? Note every RDO user would hit this issue on a fresh system otherwise

I was hit by packstack failure to install EPEL in December 2013. Then I was hit by packstack failure to resolve dependencies. I just shared manual fix been done to make packstack happy as a worst case scenario.

Try manually installing django & nodeJS on CentOS and then re-run packstack. Earlier it worked for me and installation was successful.

You don't need node.js to run Horizon at all.