Storwize/SVC: Use VolumeDriver's copy vol<->image.

Inherit the VolumeDriver copy volume<->image functions. This patch also
creates a new SanDriver class which is protocol-agnostic, and leaves
the SanISCSIDriver class for drivers that wish to use it.

Change-Id: Iad5cb030545ef7f4e01d006f1e63cdeeec713bf6
GPFS stub calls to truncate and dd in unit tests

gpfs unit tests had calls to truncate, and allowed
driver call to dd and df.  Modify tests to stub these
out.

Also remove references to image_path.  This was a temp file
created using truncate that is no longer used.

Change-Id: I437b56505ba025b256b051dfaf43d8124005f854
Fixes: bug #1203075
Merge "Storwize/SVC: Use VolumeDriver's copy vol<->image."
Merge "GPFS stub calls to truncate and dd in unit tests"