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: Ibb48fab8084962a0983c2a5260ae97b53eee8f4d
Signed-off-by: aticig <gulsum.atici@canonical.com>