This branch looks very good Matthew, thank you for this work and for
investigating this ssh stuff!
I wrote a lot of comments, but many of them are just nice to
have/minors/do-whit-them-what-you-want.
LGTM with (possible) changes.
QA ok with some comments (about the initial agent check and the SIGINT
handling).



https://codereview.appspot.com/39610049/diff/40001/quickstart/app.py
File quickstart/app.py (right):

https://codereview.appspot.com/39610049/diff/40001/quickstart/app.py#newcode98
quickstart/app.py:98: utils.start_ssh_agent()
It seems an OSError raised from this call would crash the application.
Also, this always starts the agent, even if the user has one already
running. Do we have a way to avoid that? This could also be tackled in
another card.

https://codereview.appspot.com/39610049/diff/40001/quickstart/app.py#newcode108
quickstart/app.py:108: 'keys yourself [s], or cancel [C]? ')
Maybe ^C here too? <shrug>

https://codereview.appspot.com/39610049/diff/40001/quickstart/app.py#newcode114
quickstart/app.py:114: raise ProgramExit('If you would like to create
the keys yourself, '
This is displayed as an error, but I am not sure this is an error.
sys.exit(msg) could be used instead.

https://codereview.appspot.com/39610049/diff/40001/quickstart/app.py#newcode120
quickstart/app.py:120: def check_ssh_keys():
Very nice commented function, thank you.

https://codereview.appspot.com/39610049/diff/40001/quickstart/app.py#newcode124
quickstart/app.py:124: in some way.  Raise OSError (via
utils.start_ssh_agent) if we couldn't
I don't see the start_ssh_agent call here. Is this still true?

https://codereview.appspot.com/39610049/diff/40001/quickstart/app.py#newcode131
quickstart/app.py:131: retcode, output, error =
utils.call('/usr/bin/ssh-add', '-l')
error does not seem to be used here and can be safely replaced with _

https://codereview.appspot.com/39610049/diff/40001/quickstart/app.py#newcode150
quickstart/app.py:150: raise
ProgramExit(msg.format(error).encode('utf-8'))
You can avoid encoding ProgramExit messages <shrug>.

https://codereview.appspot.com/39610049/diff/40001/quickstart/app.py#newcode164
quickstart/app.py:164: while not check_ssh_keys():
^C produces a traceback. It's not that important, but we can easily
avoid it with something like:

     try:
         while not check_ssh_keys():
             print('.', end='')
             sys.stdout.flush()
             time.sleep(3)
     except KeyboardInterrupt:
         sys.exit('\nquitting')

https://codereview.appspot.com/39610049/diff/40001/quickstart/app.py#newcode166
quickstart/app.py:166: sys.stdout.flush()
Interesting, why is this required? A comment would be nice.

https://codereview.appspot.com/39610049/diff/40001/quickstart/app.py#newcode185
quickstart/app.py:185: if retcode > 0:
Is it ok if retcode is < 0? Can this happen?
If not, we can just check "if retcode:".

https://codereview.appspot.com/39610049/diff/40001/quickstart/app.py#newcode188
quickstart/app.py:188: if retcode > 0:
Same as above.

https://codereview.appspot.com/39610049/diff/40001/quickstart/tests/test_app.py
File quickstart/tests/test_app.py (right):

https://codereview.appspot.com/39610049/diff/40001/quickstart/tests/test_app.py#newcode240
quickstart/tests/test_app.py:240: class TestCheckSSHKeys(
Nice tests!

https://codereview.appspot.com/39610049/