Mock processutils.execute properly in test_ibmnas

test_delete_snapfiles and test_delete_snapfiles_nas_gpfs were
not properly mocking out cinder.openstack.common.processutils.execute .
This was causing the unittests to prompt for a sudo password during
unit test on some systems.

This change mocks out processutils.execute to return what is expected
for this test case.

Change-Id: I67f26cfd3157813758de7785157a21ebe5230833
Closes-bug: 1361287
Merge "Mock processutils.execute properly in test_ibmnas"