[Plugin] remove add_cmd_outputs()

Remove add_cmd_outputs() and allow add_cmd_output() to accept a
list of commands to invoke. Add a check and warn via the logs if
a plugin attempts to use 'root_symlink' or 'suggest_filename'
with a command list of length > 1.

Fixes #301.

Signed-off-by: Bryn M. Reeves <bmr@redhat.com>
[Plugin] remove add_copy_specs()

Merge add_copy_specs() into add_copy_spec() and treat any strings
passed to the new method as though they were a single item list.

Mirrors the prior change to add_cmd_outputs().

Fixes #301.

Signed-off-by: Bryn M. Reeves <bmr@redhat.com>
[sos] bump version to 3.2

Signed-off-by: Bryn M. Reeves <bmr@redhat.com>
[utilities] invert sense of six.PY2 test

Old versions of six do not include a 'PY2' attribute leading to
an exception in sos_get_command_output(). Invert the sense of the
test and check that six.PY3 is false instead as all versions of
the module include this attribute.

Signed-off-by: Bryn M. Reeves <bmr@redhat.com>
[archive] invert sense of six.PY2 test

Old versions of six do not include a 'PY2' attribute leading to
an exception in sos_get_command_output(). Invert the sense of the
test and check that six.PY3 is false instead as all versions of
the module include this attribute.

Signed-off-by: Bryn M. Reeves <bmr@redhat.com>