sysctl_attachment chokes on invalid character when run on Calxeda

Trying to run checkbox on the Calxeda node, we can see in the output that there is a traceback generated upon executing the sysctl_attachment job:

Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/checkbox/reactor.py", line 74, in fire
    results.append(handler(*args, **kwargs))
  File "<string>", line 55, in prompt_test
UnicodeEncodeError: 'ascii' codec can't encode character '\u2019' in position 3693: ordinal not in range(128)

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/usr/lib/python3.2/logging/__init__.py", line 937, in emit
    stream.write(msg)
UnicodeEncodeError: 'ascii' codec can't encode character '\u2019' in position 3877: ordinal not in range(128)
Logged from file reactor.py, line 89

I traced this down to an invalid character in sysctl.conf, where a stylised apostrophe is being used rather than the basic '. The output of this command needs to be filtered to remove these characters - that is if we don't finally bite the bullet and address this issue in the inner workings of Checkbox rather than repeating the fix for every job that gets affected.