Update RHEL7 element since release

This patch updates the RHEL7 element to work
properly now that it has changed from beta to
release status.  It includes a repo update to
locate os-config rpms not in epel yet and an
update to the latest cloud image name.

Change-Id: I05607d62193c8f8fb1c1bece9872d456b19c5a08
Revert "Cleanup apt cache after grub install"

This calls chroot as a non-root user, which will always fail with
EPERM.

This reverts commit ab2d1a31f2177b26aada075201712b0061a1b771.

Change-Id: Ie674fef694ad66e1ebc22083dec2a0bc34371e7b
Merge "Revert "Cleanup apt cache after grub install""
Merge "Update RHEL7 element since release"