Fix Bug 2282: Can't instantiate NS using a pre-existing volume

    Add the posibility to return an empty dictionary to `find_persistent_root_volumes()` method
    so the NS instantiation doesn't fail while using a pre existing volume.

Change-Id: Iecd7041b2a202332acecd45a3e899d82be613ef7
Signed-off-by: Luis Vega <lvega@whitestack.com>
Signed-off-by: garciadeblas <gerardo.garciadeblas@telefonica.com>