stages/dnf: support --exclude

This allows given packages to be excluded from the transaction. This
is useful if you want to install a group with certain exceptions.

A common thing to do in kicktstart files is:
```
rm -f /boot/*-rescue*
```

By instead excluding the dracut-rescue-config package we end up
with:
```
"deleted_files": [
  "/etc/kernel/postinst.d",
  "/usr/lib/dracut/dracut.conf.d/02-rescue.conf",
  "/usr/lib/kernel/install.d/51-dracut-rescue.install",
  "/boot/initramfs-0-rescue-ffffffffffffffffffffffffffffffff.img",
  "/boot/vmlinuz-0-rescue-ffffffffffffffffffffffffffffffff"
],
```

Signed-off-by: Tom Gundersen <teg@jklm.no>

samples/base-qcow2.json

@@ -23,6 +23,9 @@
           "qemu-guest-agent",
           "xen-libs",
           "langpacks-en"
+        ],
+        "exclude-packages": [
+          "dracut-config-rescue"
         ]
       }
     },