Correct a unit test failure that crept into trunk

At some point, the "nova list" command started returning an
empty table when no results were obtained, while the
tests.test_shell.ShellTest.test_password test was expecting
a single newline.  Whichever commit caused this change in
behavior somehow made it past the gate without forcing the
test to be updated.

This commit fixes the issue by changing the output that
test_shell.test_password is expecting.

Fixes bug 1166464.

Change-Id: I57636b4a1e525c440925caba0bbb51bbcd94b080