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 "GPFS stub calls to truncate and dd in unit tests"