Fix Bug 2012 use existing volumes as instantiation parameters

This fix parses the vdu instantiation parameters regarding with the persistent
volumes and than RO can process it.
Additionally corrected some Black and Flake8 errors.

Change-Id: If6fb41684555364914af52af334d70f794f133a2
Signed-off-by: aticig <gulsum.atici@canonical.com>