In Iedf0d4f829e46ca64c3f4fc6a7dfee54d9b0605b we added support for driver
capabilities in glance_store. That same change also used (mutable)
module globals to check driver capabilities. By moving the capability
definitions inside a subclass of IntEnum we retain all of the previous
functionality with the guarantee that the enum values will not change
once defined.
Change-Id: I13c7bd507252e793b2c790cc9b77c7b8db432d02