Update EPEL version and package manager call for RHEL-based distros

[Victoria Martinez de la Cruz]

Current script would install EPEL 6.8. It would be nice to bump to a newer version.

Plus, script uses yum. This is fine for Centos/RHEL <= 7 and Fedora < 22, but latest Fedora versions, Fedora >= 22 use dnf.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/355088

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/355088
Committed: https://git.openstack.org/cgit/openstack/manila-image-elements/commit/?id=e2e7ef98d81867565091df8481d61a9c76d83a72
Submitter: Jenkins
Branch: master

commit e2e7ef98d81867565091df8481d61a9c76d83a72
Author: Victoria Martinez de la Cruz <email address hidden>
Date: Fri Aug 12 18:12:58 2016 -0300

    Fix package manager call for RHEL-based distros

    Assuming users will be working with latests versions
    for RHEL, Centos or Fedora, the requirement for EPEL
    repo is not longer needed. This requirement was due
    to argparse not being present in standard repos, something
    that does not hold anymore.
    For this reason, this patch set removes the installation
    of the EPEL repo.
    On the other hand, this patch set also makes an extra check
    to use dnf when this script is run on Fedora. Since Fedora 22
    dnf has been the default package manager instead of yum.

    Change-Id: I9fcd47a0bf9302506949fa56efb42b77552710f6
    Closes-Bug: #1611527