This contains backports for code changes made during in-tree driver
DiskAdapter parent class development [1].
- Forces subclasses to implement capacity methods
- Marks abstract methods/properties in the parent class
- Introduces a fake disk adapter to allow testing with abstract methods
and properties.
[1] https://review.openstack.org/#/c/549053/
Change-Id: I1f8ebe64571c009020be7f3888167e3450d2fd4d