Don't set sqlite_db default

Keystone doesn't use oslo.db's sqlite_db option so there's no need
to set the option. Setting the default was required with
oslo-incubator's db code but it's now optional so we can remove it.

Change-Id: I7a012f40f924f83b878ee325f8ca26bdbdb9ac1d
Merge "Don't set sqlite_db default"