Cleanup/move code in Storwize Driver

Cleanup/move code in Storwize Driver

This patch updates the name of helper.py to storwize_svc_common.py.
In addition, ssh.py has been collapsed into storwize_svc_common.py
to reduce redundant code. Unit tests were also updated to reflect
these updates.

This patch is the first of a series of patches with the aim of
bringing the storwize_svc driver in line with other san based
Cinder drivers.

Co-Authored By: Kendall Nelson <kjnelson@us.ibm.com>
Co-Authored By: Slade Baumann <baumann@us.ibm.com>

Partially implements: blueprint refactor-storwize-driver-for-mitaka

Change-Id: I7725a008105ae54e49f90c74b32aa11713e983c9