CoraidDriver: support users that are not admin

Add support to the CoraidDriver to specify a subgroup (with admin
privileges).
Coraid storage orchestration allows you to allocate portions of your
SAN to specific users/groups.
The driver should support this and not assume logging in
as "admin" (equiv to "root").

Will add another parameter to cinder.conf, "coraid_group",
and if present, the driver will authenticate using the specified group
as well as the already available coraid_user parameter.

Fixes bug 1157354

Change-Id: I0d165955126adabce788ebdacd3a14a08edc8d8f
(cherry picked from commit 0a0251e695ed600ea586233b9654aedf4dab79e6)