Merge lp://qastaging/~nobuto/charm-helpers/execd into lp://qastaging/charm-helpers

Yes, its unfortunate how much 'ignore errors by default' is used in charmhelpers when that is never the right thing to do for charms.

I've got one minor inline comment. We need at least one of the tests to call execd_run without the die_on_error argument so we can ensure the default is set correctly.

Note that reactive charms have this functionality built in (lib/charms/layer/execd.py), and its behaviour matches what is proposed here.

Reflected review comments, but I found the test is failing with Python 3. I don't know how to handle this, but will look into it.

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FAIL: tests.payload.test_execd.ExecDTestCase.test_execd_run_logs_exception
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testtools.testresult.real._StringException: Traceback (most recent call last):
  File "/home/nobuto/dev/charm-helpers_execd/.venv3/lib/python3.5/site-packages/mock.py", line 1201, in patched
    return func(*args, **keywargs)
  File "/home/nobuto/dev/charm-helpers_execd/tests/payload/test_execd.py", line 142, in test_execd_run_logs_exception
    log_.assert_called_with(expected_log)
  File "/home/nobuto/dev/charm-helpers_execd/.venv3/lib/python3.5/site-packages/mock.py", line 835, in assert_called_with
    raise AssertionError(msg)
AssertionError: Expected call: log('Error (1) running /tmp/tmp11nk64mu/exec.d/basenode/charm-pre-install. Output: stdout_from_pre_install\nstderr_from_pre_install\n')
Actual call: log("Error (1) running /tmp/tmp11nk64mu/exec.d/basenode/charm-pre-install. Output: b'stdout_from_pre_install\\nstderr_from_pre_install\\n'")

execd_run() is doing "subprocess.check_output(submodule_path, stderr=stderr)", which returns a byte string rather than text. Change that to "subprocess.check_output(submodule_path, stderr=stderr, universal_newlines=True)" or similar to have it return the output as a standard text string (unicode).

Thanks for the kind comment. I have pushed the change.