oeqa/runtime/cases/rpm.py: change to use base-passwd-doc

oeqa/runtime/cases/rpm.py: change to use base-passwd-doc

The test cases assume that rpm-doc is built out, as it says it its
comment. This is not always true. And it sometimes results in
following error.

  |     cls.tc.target.copyTo(test_file, dst)
  | UnboundLocalError: local variable 'test_file' referenced before assignment

Change to use base-passwd-doc, as this package is more likely to be
built out than rpm-doc.

Signed-off-by: Chen Qi <Qi.Chen@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>