uperf_build hangs if EPEL isn't already installed

[frival]

The uperf_build script tries to install EPEL with yum install epel-release.noarch, but if it's not already installed that will hang waiting for input from the user which it'll never get. Add a -y to the command so we can keep marching on. This, as far as I can tell, will only ever happen on network client systems as we pretty much always have already installed EPEL on systems we're targeting with Zathras.