Remove dependency on oslo-log
Use exception
Move exceptions out of common and add backends.py
Remove old scrubber options
Update functions signatures w/ optional context
Add a way to register store options

This commit adds a way to register store options by iterating over the
enabled stores and loading them. Since this may leave the stores in a
"unconfigured" mode, the `create_stores` method now calls `configure`
for each store and the register method overwrites them in the SCHEME
dict.
BugFix: define scheme outside the `try` block
BugFix: Point to the exceptions module
Remove deprecated _schedule_delayed_delete_from_backend function