Fix Bug 2041: Backup VDU interfaces for healing features

If a VDU is manually deleted in Openstack, the RO overwrites the
interfaces as an empty list. However, healing features (10909 and 10923) need the
interfaces information to heal the VDU again. This fix backups the interfaces of
VDU under interfaces_backup keyword.

Change-Id: I17ea28c2c4f486d0bbecdcbc5005413222bfeed9
Signed-off-by: aticig <gulsum.atici@canonical.com>