Migrate to flake8.

Fixes bug 1172444

Change-Id: Ieca721663aea2fd31753df4abfb5b01a7145b26a
Tweak volume commands and add k=v argparse action

Basic cleanups:
* change metadata to property
* add new KeyValueAction to parse the property options
* multiple properties can be set using multiple --property args
* consistent formatting
* do lookups for volume args

Change-Id: Ib6c43f01ad46b395aee8c61e886f42e2a5f5573e
Merge branch 'tweak-volume' into dt-test