Use $YUM instead of direct calls in more places

A few places we either assume centos uses "yum" directly, or have
switching based on the distro type.

In both cases, we can use ${YUM} directly to avoid ambiguity

Change-Id: I71095a9bd1862f8956b5982fbbb3e1d213926c14

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

@@ -8,7 +8,7 @@ set -o pipefail
 
 if [[ ${DISTRO_NAME} =~ "centos" ]]; then
     # Centos has "epel-release" in extras, which is default enabled.
-    yum install -y epel-release
+    ${YUM} install -y epel-release
 else
     # For RHEL, we have to scrape the download page to find the latest
     # release and install that
@@ -34,7 +34,11 @@ else
 fi
 
 if [ ${DIB_EPEL_DISABLED:-0} -ne 0 ]; then
-    yum-config-manager --disable epel
+    if [[ ${YUM} == "dnf" ]]; then
+        dnf config-manager --set-disabled epel
+    else
+        yum-config-manager --disable epel
+    fi
 fi
 
 DIB_EPEL_MIRROR=${DIB_EPEL_MIRROR:-}

diskimage_builder/elements/rpm-distro/pre-install.d/01-override-yum-arch

@@ -28,7 +28,7 @@ else
     exit 1
 fi
 
-if [[ $DISTRO_NAME == "fedora" ]]; then
+if [[ ${YUM} == "dnf" ]]; then
     mkdir -p /etc/dnf/vars
     echo $basearch > /etc/dnf/vars/basearch
     echo $arch > /etc/dnf/vars/arch
@@ -43,5 +43,5 @@ if [[ ${DISTRO_NAME} =~ "centos" ]]; then
     else
         contentdir=altarch
     fi
-    echo $contentdir > /etc/yum/vars/contentdir
+    echo $contentdir > /etc/${YUM}/vars/contentdir
 fi

diskimage_builder/elements/yum/post-install.d/99-reset-yum-conf

@@ -6,7 +6,7 @@ fi
 set -eu
 set -o pipefail
 
-if [[ $DISTRO_NAME == "fedora" && $DIB_RELEASE -ge 22 ]]; then
+if [[ ${YUM} == "dnf" ]]; then
     cfg=/etc/dnf/dnf.conf
     cachedir=/var/cache/dnf
 else

diskimage_builder/elements/yum/pre-install.d/01-yum-keepcache

@@ -6,7 +6,7 @@ fi
 set -eu
 set -o pipefail
 
-if [[ $DISTRO_NAME == "fedora" && $DIB_RELEASE -ge 22 ]]; then
+if [[ ${YUM} == "dnf" ]]; then
     # dnf is slightly different; doesn't have these by default
     cfg=/etc/dnf/dnf.conf
     echo "keepcache=1" >> $cfg