Ensure yum-utils is installed in epel element

We are running yum-config-manager/dnf config-manager in the epel
element. Even though the yum-utils package is declared in the
package-installs.yaml file, the package-installs pre-install.d
script is executed after the one in the epel element, so image
build fails.

This commit ensures yum-utils or dnf-plugins-core are installed
before running the command.

Change-Id: Ib292b0b2b31bd966e0c5e8f2b2ce560bba89c45c

diskimage_builder/elements/epel/pre-install.d/05-rpm-epel-release

@@ -35,8 +35,11 @@ fi
 
 if [ ${DIB_EPEL_DISABLED:-0} -ne 0 ]; then
     if [[ ${YUM} == "dnf" ]]; then
+        rpm  -q dnf-plugins-core || dnf install -y dnf-plugins-core
         dnf config-manager --set-disabled "epel*"
     else
+        # Cannot rely on package-installs, it is executed later
+        rpm  -q yum-utils || yum install -y yum-utils
         yum-config-manager --disable "epel*"
     fi
 fi