unattended_install: Make image_backup = yes by default

Meaning successful unattended installs to create backup
copies, which prevents non-jeos runs to backup bad
images at the beginning of tests and apply them on
subsequent tests. People running unattended installs
on their grids can override that param if they want
anyway.
Signed-off-by: NLucas Meneghel Rodrigues <lmr@redhat.com>

tests/cfg/unattended_install.cfg

 - unattended_install:
     virt_test_type = qemu libvirt
     type = unattended_install
+    backup_image = yes
     start_vm = no
     kill_vm = yes
     kill_vm_gracefully = yes
@@ -115,6 +116,7 @@
         - import:
             virt_test_type = libvirt
             no cdrom, url, nfs, remote_ks, pxe, kernel_initrd
+            backup_image = no
             cdroms = ""
             floppy = ""
             timeout = 180