qa/workunits/smb: make the smb_cfg fixture module scoped

qa/workunits/smb: make the smb_cfg fixture module scoped

This means the file will only be read when pytest changes modules.
This also allows this fixture to be used with other fixtures at the
module or scope "higher" than the function scope.

See: https://docs.pytest.org/en/stable/how-to/fixtures.html#fixture-scopes

Signed-off-by: John Mulligan <jmulligan@redhat.com>