Fix Bug 1911 Root disk cannot be a Persistent Volume

By this fix, VNFs could have persistent root disk if the disk type is defined as persistent-storage.
Besides, the problem of creating unnecessary empty persistent volume is solved.

Change-Id: Id032701bba4ce71080c338bc7f112261ec98861e
Signed-off-by: aticig <gulsum.atici@canonical.com>