Can't use `.execute` expecting non-utf-8 output

The instance `execute` method calls ends up (in `_ssh`) calling `.decode('utf-8')` in the output of the command which is executed. Because decode is being strict, this raises an error if the command returns non-utf-8.

Although I understand UTF-8 can be the default decoder, we could make use of an option either to ignore/replace errors, or to specify a different decoder.