Fix bug 1699 to add persistent-storage type to vdu

This change adds new augment file persistent-storage.yang, which adds a
new type of storage under 'vnfd/virtual-storage-desc/type-of-storage'.

Specifically, the changes adds a new identity to the existing base defined in
'etsi-nfv-descriptors/storage-type'.

Change-Id: Ica32cbb995434c2cc3a482b1cceb3848f293fd96
Signed-off-by: garciadeblas <gerardo.garciadeblas@telefonica.com>