RHEL7: Fixes EPEL repository package location

`salt-bootstrap` tries to install the `epel-release-7-1.noarch.rpm` from the url `http://download.fedoraproject.org/pub/epel/7/${EPEL_ARCH}/epel-release-7-1.noarch.rpm`, which is 404'ing: [ppc64](http://download.fedoraproject.org/pub/epel/7/ppc64/epel-release-7-1.noarch.rpm), [x86_64](http://download.fedoraproject.org/pub/epel/7/x86_64/epel-release-7-1.noarch.rpm).

The correct one is: `http://download.fedoraproject.org/pub/epel/7/${EPEL_ARCH}/e/epel-release-7-1.noarch.rpm`: [ppc64](http://download.fedoraproject.org/pub/epel/7/ppc64/e/epel-release-7-1.noarch.rpm), [x86_64](http://download.fedoraproject.org/pub/epel/7/x86_64/e/epel-release-7-1.noarch.rpm).

This pull request fixes the regression from PR #453.