Comment 1 for bug 1751051

I think the issue is:
a.) there is no default locale set in the subiquity installed system.
b.) python3 subprocess is doing a 'decode' for each argument in the
command list.
python2 default encoding *is* supposed to be based on the environment [1],
but python3 default encoding is not. python3 is supposed to be utf-8.
In the trace above we are down in C code where it is clearly doing 'ascii'
encoding.

 [1] https://docs.python.org/2/library/sys.html?highlight=getdefaultencoding#sys.getdefaultencoding
 [2] https://docs.python.org/3/library/stdtypes.html?highlight=decode#str.encode

You can see the problem generally below. I only use 'json' as a convienent
way to pass in utf-8 characters. You can see that either unset LANG
or LANG=C causes the issue.

I guess I never thought that subprocess would be converting an argument
list of strings to bytes. That does make some sense.

So I think there are actually two changes:
a.) subiquity (via either curtin or cloud-init) should be setting a utf-8
default locale (all ubuntu generally do that). I'm not sure why the image
being installed didnt have one set.

b.) cloud-init's subp should probably just do the conversion to bytes
of whatever it gets as an argument list for the command, and always assume
that strings are to be encoded as utf-8.

$ cat go.py
#!/usr/bin/python3
import json, subprocess, sys
cmd = json.loads(sys.argv[1])
print("cmd=%s" % [x.encode("utf-8") for x in cmd])
subprocess.check_call(cmd)

# my default lang is en_US.utf-8
$ ./go.py '["echo", "Andr\u00e9 DSilva"]'
cmd=[b'echo', b'Andr\xc3\xa9 DSilva']
André DSilva

$ LANG=en_US.utf-8 ./go.py '["echo", "Andr\u00e9 DSilva"]'
cmd=[b'echo', b'Andr\xc3\xa9 DSilva']
André DSilva

$ env -u LANG ./go.py '["echo", "Andr\u00e9 DSilva"]'
cmd=[b'echo', b'Andr\xc3\xa9 DSilva']
Traceback (most recent call last):
  File "./go.py", line 5, in <module>
    subprocess.check_call(cmd)
  File "/usr/lib/python3.6/subprocess.py", line 286, in check_call
    retcode = call(*popenargs, **kwargs)
  File "/usr/lib/python3.6/subprocess.py", line 267, in call
    with Popen(*popenargs, **kwargs) as p:
  File "/usr/lib/python3.6/subprocess.py", line 709, in __init__
    restore_signals, start_new_session)
  File "/usr/lib/python3.6/subprocess.py", line 1275, in _execute_child
    restore_signals, start_new_session, preexec_fn)
UnicodeEncodeError: 'ascii' codec can't encode character '\xe9' in position 4: ordinal not in range(128)

$ LANG=C ./go.py '["echo", "Andr\u00e9 DSilva"]'
cmd=[b'echo', b'Andr\xc3\xa9 DSilva']
Traceback (most recent call last):
  File "./go.py", line 5, in <module>
    subprocess.check_call(cmd)
  File "/usr/lib/python3.6/subprocess.py", line 286, in check_call
    retcode = call(*popenargs, **kwargs)
  File "/usr/lib/python3.6/subprocess.py", line 267, in call
    with Popen(*popenargs, **kwargs) as p:
  File "/usr/lib/python3.6/subprocess.py", line 709, in __init__
    restore_signals, start_new_session)
  File "/usr/lib/python3.6/subprocess.py", line 1275, in _execute_child
    restore_signals, start_new_session, preexec_fn)
UnicodeEncodeError: 'ascii' codec can't encode character '\xe9' in position 4: ordinal not in range(128)