Set iet_conf to nonexistent file in unit test

The Iet test in test_iscsi isn't setup to deal with
an actual iet.conf file.  The result is that if you happen
to be on a system that has an iet.conf file the test will fail
when it gets to the os.stat check.

This patch just uses a tempdir with a bogus file name that is
set as the iet_conf option to make sure the test never runs into
a situation where the file is actually there.

Change-Id: I5c8653a17250063660ca8aa369c537cc340bc4e9
Closes-Bug: #1400780
Merge "Set iet_conf to nonexistent file in unit test"