Update glance::backend::rbd options

This patch is aim to add rbd options in glance-api.conf
and also update related spec test.

Fixes bug #1221074

Change-Id: Ifcbaf15ab4a7aedb074abac42941181e53d538c4
(cherry picked from commit 6dc226bbde6a3fffb5be200df0801ff3e91ceecf)