Storwize/SVC: allow setting of I/O group

Allow setting the I/O group via config option or volume type. Also
updated comments and checks regarding iSCSI multipath, which is in Nova
and is not controlled by the driver.

Fixes: bug 1215363

Change-Id: Ia67fcb9991e21990cc81830145b8fd934a23488e
Merge "Storwize/SVC: allow setting of I/O group"