Add entry-point for oslo.config options and update registering logic

1. The change exposed all options of store and driver by oslo.config
entry-point.
2. Refactor store and driver options registering logic to leverage this
unified option discover function.

Change-Id: Ibbd0f27abd0c0eb9a88d8d30de1e89264a11f53d
Signed-off-by: Zhi Yan Liu <zhiyanl@cn.ibm.com>
Merge "Add entry-point for oslo.config options and update registering logic"