IBM Storwize driver: Add local variable assignment to "ctxt"

* The method get_vdisk_params in helpers.py is missing a local variable
assignment for "ctxt", causing "UnboundLocalError: local variable
'ctxt' referenced before assignment. Adding the assignment should
resolve this issue.

* Add the unit tests coverage for get_vdisk_params.

Change-Id: I564b1ef8cd1b6504d5ac8c9af0bb11bf29767d9a
closes-bug: #1379654
Merge "IBM Storwize driver: Add local variable assignment to "ctxt""