dracut/30ignition: Add packages to delete users/groups

This PR adds two packages namely `groupdel` and `userdel`
to delete the existing users/groups from the base OS.
xref: coreos/ignition#738

dracut/30ignition/module-setup.sh

@@ -25,6 +25,7 @@ install() {
     # present
     inst_multiple -o \
         groupadd \
+        groupdel \
         mkfs.btrfs \
         mkfs.ext4 \
         mkfs.vfat \
@@ -33,6 +34,7 @@ install() {
         sgdisk \
         useradd \
         usermod \
+        userdel \
         wipefs
 
     # Required by s390x's z/VM installation.